I guess I'll add my two cents to this. I haven't tried Solaufein yet, and I was quite surprised reading this thread about this whole issue. But, I think I have something to add.
I think that the source of the controversy on this issue is the aspect of roleplaying involved in this game. As for me, I will only ever play as male characters in any roleplaying game when given the choice. Why? I feel like they are the only characters I can identify with, simply because I am male in real life. Even if there is no element of sexuality/gender identity in a game (meaning game characters would respond identically to you regardless of your gender, there are no romances, etc.), I will still feel compelled to play a male character, because the feeling of separation between myself and the character is greater when I play a female (or alien/creature/whatever the context). Sure, I can play a female and enjoy the game if the game is good, but if there is a character creation element to the game and I am allowed to choose a gender, I just feel like I have to pick male.
With that said, let's look at sexuality roleplaying in games. The truth in real life is that many straight men will be uncomfortable if a homo/bisexual man hits on them and/or makes it known that he is sexually attracted to them. That is a fact of life: that many straight men are uncomfortable with homosexuality, regardless of their attitudes towards homosexuals, including myself. I personally think that the hatred of homosexuals that some people have is terrible, and I have no problem with being friends with homosexuals (because I do have homosexual friends), but honestly, when a homosexual man hits on me (and this has happened), I feel very uncomfortable. The reasons for, consequences of, and morality involved with this are beyond the scope of this debate, but you can see where I am going with this. If the typical straight man is roleplaying a male character in BG2, it is likely they will be made uncomfortable (or will make themself uncomfortable, depending on how you see it) with a bisexual/homosexual male NPC's propositions.
So my point is this: by addressing this issue you are addressing something that is really beyond the scope of this game-related forum. Not that you shouldn't discuss these issues, but the psychology runs very deep. More appropriately, I think we should just look at Solaufein as a fictional game character and see his apparent bisexuality as a mechanism for allowing players to play him however they wish, and leave it at that. The purpose here is enjoyment, and allowing a homosexual relationship with Solaufein to occur widens the player base in this regard. So that's my $.02.