I would love to see someone have to give reasons for being straight for once... that might be an eye opener... 
On a less joking note though, I completely agree with your sentiments. Why should they even have to explain themselves? If they're romancing you (and you're playing the same sex), it's pretty obvious that they're into you... regardless of gender. Yeesh.
It would be nice if there wasn't a need for it but...
I think in the game, they wanted to give the players a ... heads up about the dialogue they were choosing, that it would begin a same sex romance. Since the majority of gamers are hetero-normative, it only makes sense for the devs to "warn" players, so to speak. With Zevran it wasn't terribly obvious. My husband romanced him by accident. lol! 
So I think it's a reflection of our society, to be honest. In this day and age, it's still unusual for a person to have someone of the same gender come onto them. And it's even more unusual to have a person you like, reply that they too like you. Such exchanges could get ugly if they admitted their attraction to the wrong person. But it's better to be clear from the off so you don't go too far down that road, thinking it's all good only to alienate your friend later because they found out you have the hots for them. So I don't agree that anyone should have to explain their orientation but I do think we're at a point in our evolution that stills needs it to prevent misunderstandings. Otherwise, most people will just assume everyone is like them. Which completely undoes the progress of fostering open minds. IMHO, of course. 