Call me a sceptic, but Astarion seem much more like playersexual (or more like "anythingthatiswillingtoofferbloodsexual") than Halsin.
I agree that in Astarions case, bisexuality or homosexuality might fit the character. But again, the 'character' should decide himself. Don't write a character to fit a spot or some quota. Let the characters decide what they want Make them 3 dimensional, including players having to deal with rejection.