Jealousy can be an issue. Some partners may prefer not to know as long as you play safe. Others may want all the details. I have friends who have been together 30+ years and share everything. One is very up front about their openness (leading the other partner to often exclaim, "my partner has a big mouth").
As for straight people, of course they do. However, there is a lot of social pressure on them to live happy, monogomous married lives and keep their extra marital affairs secret. Lesbians and gay men have had, up until recently, the ability to define our own relationships. Social pressures for monogamy really haven't been there. I think that is changing, since there is a strong social implication that marriage is the same as monogamy.
My $0.02
I'm still a bit skeptical of being so laissez-faire about sexual relationships outside of the relationship. I want to clarify that I don't begrudge anyone who recognizes that this is a personal want in their relationship, and that they have taken the proper steps to make this want known to their partners AND that you two have talked it out and come to an agreement. When the partner meets this want with hesitation, uncertainty, or feeling betrayed, and the one who initiates the idea atcs indifferently or tries to plead a case for it anyway, that's disrespectful.
I recognize that standards of monogamy have changed greatly over the past few decades. Call it a Sexual Revolution, if you want. And it's also true that people are seeking each other out for companionship and enjoyment and simply not a lifelong wed-as-one commitment. If companionship includes casual dating and sexuality, or allows for casual behavior in the midst of a serious relationship, then call it as you want. Just be mindful that there's a lot of work that needs to be done in order to start calling those shots.
It's up to both partners to decide what constitutes fidelity. And it's pretty clear that some partners want to be open about what they do, so they're not afraid to give details. It's ok. As long as that standard was agreed upon, it's respectful.
I chuckled a bit when I read the end of that reply I quoted. I don't think gays and lesbians (and bis and transgendered people) were given some sort of permissiveness made alien to heterosexuals. I always thought they were held to the predominant heterosexual social standard -- that you're really meant to be with the opposite sex, you're supposed to stay together, but oh bloody hell, men are men and men will cheat and if women do it, then they're sluts.
From my own experience, I have never cheated on someone with whom I have been involved in a serious relationship. Yeah, I got close a couple of times. When I started having some fun with guys and knew that I couldn't contain that impulse, I had to be honest to my girlfriend -- ok, so I didn't say "guys" explicitly, but I said that my eyes had been wandering and that I didn't want to tell her this after the fact. I felt guilty. I didn't think it was fair to put her through that, so I ended things. And I think it takes a really brave person to consider what extramarital play might mean to them BEFORE actually doing it.
If the relationship can sustain itself, well, that's great. Am I skeptical? Sure. I couldn't be "the other man." I wouldn't do that and I recognize that's where I stand.