With some exceptions they can be obtained in various ways.
Here's those that are only obtainable in one way:
1) Footjob - only obtainable from playing HBG game with with Cream, where it's up to random chance. I think you just need to have her naked, I'm not going to dig through the .rpy's to find exact requirements, they are convoluted on this one.
2) Deepthroat - requires more than 5 blowjobs, then watch cartoons -> watch movie -> pull up dress -> finish yourself in bathroom.
3) Grind on swings/Sex on swings - Requires her to be naked and more than 10 lust, go on swings. For sex, you must've already had sex with her once elsewhere and seen grind event (so just go on swings twice if you didn't).
4) Sex on beach - Requires going to nude beach after having sex with her before, and there are two ways: 1) Steal her top while "seeing who does biggest splash" in the pool, you'll need 400 RWC or -7 moral alignment or cream being OK without a dress for that to succeed. Then suggest going to the beach if she is OK being naked. 2) Go to the beach from the town with more than 500 RWC or Cream being OK being naked. If going from the town, there's a chance you'll get even with Blaze instead.
5) Sex at movies - go see action movie on Thursday, Friday or Saturday, slipping a hand first requires 10 touch, actual sex requires her to be naked and 650 RWC.
6) Sex at store - go to cubby corner fashion store, peep on her, then tell her she's cute without the dress, she needs to be naked.
7) Sex with Blaze at nude beach - see "sex on beach" above
8) Oral from Sally at Fountain Square - just keep watching at the fountain on even days, there's no requirement.
9) Sex with Amy at her place/Threesome with Amy - Have sex with Cream, then go to park on even days, and keep going right on nature trail, eventually you'll run into Amy. Tell her that she's distracted and watch events unfold. Note that having sex with Cream prior to this is necessary to unlock threesome with Amy, not to fuck Amy. Then just call Amy and have sex with her (having Deepthroat has a slightly different scene where Cream demonstrates that to Amy).
10) Threesome with Sally - Have sex with Cream, call Sally and ask about gossip. One of the options is that she talks about Vanilla fucking like a rabbit, and you get the choice of confessing to Sally, do that. Then just call Sally over and fuck her.
11) Threesome with Knuckles - Requires having both vaginal and anal sex with Cream, call Knuckles over, put a porno and suggest having fun with Cream.
12) Threesome with Marine - I don't know if this is implemented yet.
13) Threesome with Tails - Requires having Cream naked, her done a blowjob, vaginal and anal sex. Call Tails over and suggest playing games, Cream suggests strip poker. Actual scene is random of three.
14) Dominate Tails - the infamous "rape tails" scene, requires -20 moral alignment and Cream's bikini top being stolen (and her being OK naked). Locks out Tails for the rest of the playthrough (he refuses to answer Sonic's calls afterwards).
15) Dominate Rogue - Cream must be OK being naked, requires -10 moral alignment and choosing to not trust Rogue. Note that this results in the pornstore being closed for 3 days. A more extreme scene requires -20 moral alignment, and completely locks out the pornstore (Rogue is... wrecked... to say the least).
