Let's use the sailor example again: they prefer anal among all other sexual activities, so they'll probably ask for that every time they come in our brothel; let's say anal has double chance to be chosen from a sailor.
If you build a furniture which grants +50% service preference among customers, a sailor will choose which sexual act he wants in this way: best chance is still anal, then it comes service with a better chance and finally the other acts with normal chance.
Having all the furnitures granting +50% for all sexual acts but anal will change things like this: anal is still a sailor's most preferred act, but the other acts will have a better chance, closer to anal, so if in normal conditions a sailor preferences would be +100% anal only, with those furnitures built (all but anal) they become like if it was anal +50% (because the other acts reduce the gab by 50%, so 100-50 = 50%)
Now with all furnitures granting +50% preference for all sexual acts: it doesn't change anything because anal among everything would gain the same +50%; so we'd have 100+50= 150% for anal, 50% for the other acts means 150-50 = still 100% for anal.
Let me give you a suggest: there's a perk which allows a girl to get more tips if she has only one sexual act enabled; a virgin girl can get a perk which doubles her income; another perk allows a girl to make a client choose another sexual act. Combine all three with a single furniture which grants a bonus for clients preference for that sexual act, and we'll do tons of gold! A virgin girl with those perks and only anal activated, in a brothel having a furniture with +50% anal and serving mostly sailors (but can be allow better clients too, no problem) can be a true gold mine. And you just have to train her for anal, if you have urgence to spend AP points for other tasks than training girls.
Not every girl becomes fascinated so fast; you are lucky if you haven't met a bigoted girl yet, they take a long time training only to make them stop refusing certain sexual acts, figure out how much they take to become fascinated!
Normally a girl refusing a sexual act or being very reluctant for that, also has a phobia related to taht act. If you start training a girl who refuses sex, for example, you may notice the broken heart symbol next to the act name (she hates it): it means she has only phobias for sex; a tao symbol (she has mixed feelings) means she has both fixations and fears; a full heart means she only has fixations.
If you manage to remove all the phobias related to a sexaul act, the girl won't hate it or have mixed feelings anymore: the broken heart would disappear and she won't hate it anymore, but neither she will love it particularly, it will be just normal for her; the tao symbol will leave its place to the full heart, meaning she now loves it, no more mixed feelings.
All of this will change her preferences too: once there are no more phobias for an act, the girl won't refuse to train for it and her preference will increase faster; once a girl becomes fascinated, she will likely accept both training and whoring in that act (but there's still a tiny chance she may refuse to work, due to a bad obedience test). And yes, sexual preference influences an obedience test related to that act.
One last thing: every sexual act is linked to a specific job: service-waitress, sex-masseuse, anal-dancer, fetish-geisha; sometimes a girl's client may ask her to do lewd things with her even if she's just working and not whoring, if it happens a high girl's preference for that act may bring her to accept her client's request and have a sexual intercourse with him. Girls who have double shift perk (the work half shift and whore half shift) will always accept such requests while working and that will count as a whoring client too.