How did I unlock the anal sex? .... I always thought that an honest, steady relationship, a glass of wine and some flowers would do the trick...
As the saying goes: Different strokes for different folks. All of that is variable to the individual.
Laura, for example is pretty quick to get to try anal with you in my experience, but still holds off on vaginal until you meet the criteria. This criteria is, for the record, explained via the relationship questlines. Just keep going through those, and you'll reach your prize with time and patience. The more a girl likes you, the more likely they'll be willing to let you in their pants, but they won't just arbitrarily put out
(at least none of these three will, we'll see with some of the future options). If we're talking numbers, I believe trust trumps love on certain things, but that might also just be the way it's worked out from how I play. IIRC, Laura was up for anal at around the 500 Love/600 Trust mark or so? I'll do some testing to confirm for you, assuming someone else doesn't answer. As far as during the sex minigame goes, though, much like IRL you gotta prep first.
The problem that most people seem to be running into again and again is impatience in one form or another. Either they don't read thoroughly enough and miss an explicitly stated detail (and/or assume such detail is just dialogue for dialogue's sake), or they outright don't read. The other camp are often the ones complaining about "all the needless consent", which is a far more blatant impatience.
In either case, just slow down a minute. Pay attention to what the girls are telling you. Think real hard about how you choose to act or reply. This is a Slice of Life sim with sex as a bonus, not the other way around.