The thing to remember with vampires is that they are not a totally 'alien' species in relation to humans. They are essentially human/demon hybrids with many of the desires, emotions behaviours familiar with their human counterparts....What vampires are perhaps more likely to demonstrate, is a lack of inhibition. Therefore they are more likely to engage in behaviour that is perhaps the preceding human might find taboo.
Many of the things Angelus does are not just things that Angel would find taboo, but are things that Angel would never, ever do, such as Angelus' often severe cruelty, his aesthetic and artistic enjoyment of cruelty, murder, rape etc. to name just a few obvious things. I don't think it's just a lack of inhibition that is the chief difference, but more than that. Especially when I regard Angelus and Angel, I see nothing but striking differences in who and what they are. They share far less in common than otherwise. Angelus' personality is also extremely different than Angel's. They're nothing alike to me.
So on the topic of Angel's sexuality, I likewise don't think anything that Angelus does or says or likes is relevant at all to what Angel would do or say or like. Like you, I'm inclined to believe that Angel is heterosexual...however, if he did turn out to be bisexual,
it would be for reasons that had absolutely nothing to do with Angelus' reasons. Angel is human and Angelus is not, and like I've said already, I don't find them to be at all alike in any way.
I would extend the same observation to Druscilla, judging from what we saw of what she was like before she became a vampire and after: two completely different people entirely.
But then there is Spike and many other vampires, who blur that line in ways Angel/Angelus do not. To be honest, I don't think it was something that the writers and creator of the show had worked out completely, at least not to my satisfaction (the nature of vampires and their relationship to the original person). There is contradictory evidence and different suggestions during the course of both shows, and some vampires are confusing in this context. This isn't meant to be a criticism of Spike, but Spike doesn't really change all that much in my opinion (not in fundamental ways) after he gets his soul back. I always wondered why Spike didn't radically change back into the meek and sensitive man he was before he became a vampire. That would have made more sense to me. Instead, he doesn't really seem to change much at all.
Anyway, I've gone on long enough.
