So, to answer the questions on how the 69 pose unlock works, because it's a bit of a hassle and not well explained anywhere. You can skip to the end of this post of a simple set of instructions that doesn't explain the actual code situation behind how it works.
As has been noted, the easy to understand requirement is that you need to Either be receiving a blowjob or licking the girl's pussy. That said, it's a bit more complicated than that. The scene that needs to be called is the [GirlName]_69_Intro, which can be found in each girl's [Name]_Scenes.rpy (at least for girls that have 69 scenes already which are Betsy, Doreen, Kitty, and Rogue). That's the end of the situation, not the start. So, how do we get there?
Well, the scene gets called from a method named Escalation(Girl=0) found in the "script Functions.rpy" (to be clear, the "Girl=0" is the function asking for a value to be passed in when it's called, or defaulting the value to0 if none is passed in). This is a method that handles girls deciding to change actions on their own if the player spends a while on a given action. Escalation in turn is called near the end of every loop after choosing to continue an action during sex. So, now for how that gets us to the 69 unlock.
The first hurdle is Escalation's return initial if statement that decides if there's a chance of the girl escalating things or if it will just go back without doing anything. To get past this you need the variable Cnt to be greater than or equal to 10, ThreeCount to be greater than Round, and Girl.Forced to be false.
- Cnt is a variable that counts how many rounds you have performed an action (not counting initially starting a given action). It is set to 0 when you start a new sex act, and increases by 1 every time you continue (including changing specifics like having the girl adjust how she is giving you a blowjob), which happens before the call to escalation (in other words, you need to keep going 10 times to meet this requirement. Overall, this is fairly strait forward, if time consuming.
- ThreeCount is a variable that primarily pertains to threesomes, while Round is a counter for how long you have before the game will force you to go to the next timeslot (Morning>Afternoon>Evening>Night>Morning). At the start of a timeslot, Round will equal 100, and any time passing will cause it to reduce until it reaches 0 at which point you are forced to go to the next time slot. ThreeCount will usually be 100 if you are in a sex scene with a single girl. That said, if an attempt at escalation is made that gets past the initial check, ThreeCount will be set to 0 and stay there, preventing any future attempts that session that sex session (at least without cheating). If however you're in a threesome, the value can be set higher again. When the girl who isn't the focus chooses an action, ThreeCount will be set to a value 2 to 4 lower than the current Round value. It is then used by the non-focus girl to count down to her potentially switching up her actions on her own. However, when she does so, it will again reset the value to one slightly lower than Round, so it can give more attempts, but you'll only get a shot every few rounds.
- Girl.Forced being false just means you haven't forced the girl to do something recently.
So, now that you got past the initial hurdle of actually having the girl potentially escalate things, there's the actual requirements to call the [GirlName]_69_Intro. Fortunately, this is pretty simple and strait forward. You need to have Trigger be "blow", "cun", or "lick pussy", and for the girl's lust ro be greater than or equal to 30. Trigger is set based on what sexual action you are performing with the girl. "blow" is getting a blowjob as a male character, "cun" is receiving oral as a female player character, and "lick pussy" is performing oral on the girl. If those requirements are met, we get to the actual 69_Intro scene.
So, in theory a girl could have different requirements from the others in the future. In practice, they all have the same requirements at this time. First, there is a check to see if you've already gotten the intro with this girl. If so nothing happens and the sex scene continues as normal (with ThreeCount set to 0). You can however just pick the 69 option in the menu. If you haven't had the intro yet, the checks will depend on how you got there:
- If you are licking the girl's pussy, you need to have [GirlX].LickP greater than or equal to 3 (this means you've initiated licking her pussy at least 3 times ever in the given save file, regardless of how long you spent licking it each time. You could simply start licking her pussy, immediately stop, then start again 3 times to meet this requirement).
If you meet that requirement, there is a second check, which can be passed either by having received at least one blowjob, or having received oral from the girl at least one time, or having an approval of at least 1300 (approval is the summed value of Love, Obedience, and Inhibition) with her having seen your penis (or vagina) at least once in the past. If this is true, you will initiate the 69 intro and unlock the pose for future sex sessions.
- If you are receiving a blowjob or oral sex from the girl, it's easier. There's a check to see if you've received at least 3 blowjobs from the girl or received oral from the girl as a female at least 3 times (note you could for instance have been a girl, gotten oral 3 times, turned into a guy, then pass this check on the first time you get a blowjob, or the reverse. Easier however is to just pass the check for 3 times getting oral sex as the same sex to trigger the scene with). After that, there is no further check to see if you've ever licked her pussy or if she likes you enough to consider letting you (there used to be a check to see if you'd done so at least once, and a comment in the code still references said check, but it has been removed). At this point you've successfully triggered the 69 intro scene.
TLDR: To easily get it quickly, get at least two blowjobs (or cunnilingus) from the girl across all times having sex with her on the current save (duration of the blowjobs and achieving orgasm is irrelevant, all that matters is starting them). Alternatively, get her total stats to 1300 and let her see your genitals, or get at least one blowjob or cunnilingus session from her and lick her pussy at least two separate times in all times having sex with her on your current save. Start that same action you've picked to do twice already again, and keep going at 10 times (if either of you orgasm, keep going rather than returning to the sex menu). Ensure the girl's lust is greater than 30 on the 10th time. If another girl has joined in, you may need wait longer by an uncertain amount, so I suggest trying to avoid having that happen.
Hopefully this is helpful for anyone trying to figure out how to trigger the 69 scenes with a girl.