The only society I've considered this for so far is one that I've built as a Pathfinder setting, so that should colour it a little bit :)

For the most part, this society is relatively sexually laissez-faire. I've passed on the whole "humans sleep with everything" trope and decided to go with a more "everything sleeps with everything" idea: that is to say, when someone says "I'm a half-elf" it's not just assumed that the other half is human. People don't describe themselves that way - they'll say "half-elf-half-gnome" or what have you, specifying both sides of their parentage.

There's a lack of taboo concerning any pairings between members within a given race - humans, for example, don't give a crap what colour your partner is. In the city, the only taboos come about if a 'civilized' race ends up mating with an 'uncivilized' one, or if the two people are from races of different size categories (essentially an easy way to demarc what's usually considered too much of a size difference). No one will blink an eye at the halfing-gnome couple, but they'll give you the hairy eyeball if you're a human walking down the street holding hands with a halfling (because of the size difference) or a gnoll (size difference and gnolls are considered uncivilized). Outside of the city, interracial pairings are rarer in large part because towns tend towards being primarily populated with single races. They'll look at your weird if you're a human holding hands with an elf, but (generally) don't get too judgmental about it (though they're much quicker and harsher to judge those outside the "acceptable" racial pairings specified above).

Gender roles exist, but aren't considered so much a set of rules as they are as set of assumptions. Looking at a woman, most people would assume she's not a soldier until she tells them she is, but she generally won't get back-talk from anyone about how that's "not a woman's place" or anything. This is reflected in relationships, too. It's assumed that men are the breadwinners/pants-wearers in a given relationship, but people (especially in the larger cities) don't care so much if that turns out not to be the case.

Homosexuality is not only NOT taboo, it's actually actively encouraged in the nobility - most noble marriages are marriages of power and convenience. There's no expectation of love at all, so it's very, very common for both the husband and wife to take other lovers. These lovers are usually taken openly, and often end up being friends with the spouse as well. It's highly encouraged for these lovers to be same-sex pairings, to make entirely certain that illegitimate pregnancy is impossible. Note that it's not unheard of for these lovers to be heterosexual pairings. It's slightly more scandalous when that happens and is considered somewhat selfish - the "common wisdom" of the time suggests that everyone is basically bisexual and that people only have mild preferences towards homo- or heterosexuality, and those who claim otherwise are generally considered to be simply selfish people who refuse to accept that they cannot always have their preferred partner. Because magical contraception is a thing and because magic can make certain of the parentage of a given child, it's never really a concern that, even if the wife in a given marriage has a male lover, her child by him (which she shouldn't be having in the first place, but mistakes with the contraception happen) would end up "accidentally" claimed by her husband. If the wife has a male lover, it's simply considered a matter of course that the child's parentage is confirmed at birth. If the child is not the father's it's usually cause for some gossip among the noble circles, but the child is otherwise treated like any other bastard child.