Eh? Past experiences definitely flesh out a character. What's Leliana without her bard background? Morrigan without being a Witch of the Wild (which has an impact on her sexuality by the way, what with the Old God baby plan and whatnot)? Anders without his Circle experience? Fenris with being a slave? Similarily, a person having a history of being with women should have an impact on any relationship with the player. Whenever it's ''oh you're the first man I've been with'' or ''you have something I like in women'' or something to that effect. Or the character being a virgin, even.
I completely fail to see how that's irrelevant to a romance, people talk about that stuff with their significant others all the time.
Okay, with the OBG, there are lesbians that get pregnant and I doubt that artificial insemination would have been available to Morrigan. So that doesn't say squat about her sexuality, other than she doesn't mind being with men. Anders and his Circle experience is barely talked about, even less to a female Hawke. He more mentions "everyone kissing everyone," a detail they could have left out and impacted his character none at all. Fenris being a slave has zero to do with his sexuality, unless you're talking about Decimus using him.
Alistair being a virgin certainly didn't impact the player much, at least not me. He was shy for about 4 lines of dialogue, then suddenly he was hailed as fantastic in bed and my PC was being told to keep it down. I'm still not seeing how making them have set sexualities fleshes out their characterization anymore than if they were playersexual/pansexual. They can talk about those experiences, but how does that make them more rounded out? I'm failing to understand this.