I'm still a little lost, how can I release new opinions to reduce these values?
When you talk to someone there will be an option for 'i try to get XY to change her/his opinion'.
When you do that a list of sub-sections open. Jobs/Personality/Sexual Acts/ Physical attraction and so on.
Picking one of those further open up several pages of possible opinion. Now those are the genuine opinions that appear on the character sheet and you can alter them to hate/dislike/undecided/like/love, one step per success from the initial state you found them to be.
Once per interaction. And it isn't automatic, whether you succeed or not is a combination of several factors. Traits, the current status of these opinions, sexual orientation, how much the person likes you how much he or she finds you attractive.. how good your social skills are. These all affect the outcome. My observation is that the opinions themselves are weighted too. So some will be harder to change by default. They aren't equal.
Sometimes you will be able to change one on the first try while other times it will take you forever. (Likely because they are governed by attributes and those differ for you as well)
More importantly, sometimes the odds are stacked against you so you have no chance to change a particular opinion unless you "undermine" it first by changing others because these are pretty much connected/overlaping to various degree.
For example in my experience by default you have a better chance to alter opinion if you first go for Following Orders under ther 'personality' category as it seemed to me that the more they are into that the more likely they comply.
How much someone is attracted to you is basically a large pool of combination of things. The sum of their opinions, some special traits they have that you can find on their character sheets (Love at first sight, Having a crush on you) which are basically just bonus or malus numbers on the overall Opinion/Attraction score.
Sexual orientation is it's own little thing. If you want to turn a gay woman into bi that is a bit different because a gay woman will have -100 on how much she find you attractive which overrides the combination of other opinions that mostly govern how much a woman likes you.
So you have to brake that down and it has it's own special discussion topic when you talk with her BUT it works very much the same way. You will have to sway her from her ways the same way you do with opinions. Keep trying untill she relents. But the odds are stacked against you, obviously, so it takes longer time and the whole "undermine" thing comes into play that i mentioned.
You see, you can try forever to simply try and directly sway a gay woman but if you suck at the skills that are offered to use on her, if your willpower/sociability is low and hers are high.. if she doesn't like you and if you haven't set her Personality to Love following orders then you will be just banging your head against a brick wall. So you have to work on these things before you have a chance.
But not all gay are made equal either

How gay a character here is an interval and not just a Gay/Not-Gay switch. This is a hidden value but lets just say that it's a scale of 5.The character sheet will simply tell she is gay. But if she is 5/5 = aforementioned brick wall.
If she is 4/5 gay you will see her attraction score will drastically shift from -100 toward the positive values and you will be able to have sex with a 4/5 already. But will only turn officially Bi when it becomes 0/5. Every decrease of this value restores the affects of the default opinions over attraction. With each decrease she will find you more and more attractive, depending how those opinons are.
What this means that if you are dealing with a gay girl of 5/5 that overrides every sexual opinion of hers. You can set her loving cocks and whatever she still won't touch you let alone sex you. Once it's 4/5, those other opinions like how much she likes the types of guy you play will immediately take effect and so you will see a gay girl changing from -100 to +8 for example if you set her heterosexual opinions high previously.
Asexual is a different beast again. I haven't figured out yet because i met my first one today the first time

I'm not even sure if they can be changed at all or you can only rape those.