Personally I don't think they(Buffy & Angel) are acting out of character.
Well, from what I've heard recently, you're alone in that opinion.
Here's why
1st - it's not like Buffy hasn't had sex with a guy that has attempted to kill her before -- remember BtVS season 6(Spike).
Well, I like to think Buffy has learned her lesson since then. Screwing evil vampires = bad. Buffy no longer has a "death wish", and she no longer believes that her death is her "gift." Buffy wants to
live.
2nd - It's not like Angel, hasn't ever done dark things, like killed a person or let a person die in order a greater number of people, or fool other evil people in to believing he's on there side -- remember Angel Season 5(Illyria & Black Thorn).
Dark things? Well, this is as dark as it gets. Yes, Angel did get Lindsey killed, Drogan as well, and allowed Fred to die to save thousands - but the magnitude of this really can't be compared to those examples. Twilight has killed hundreds of people, tortured Willow, etc etc. God KNOWS what he might have done to and of A.I. back in L.A. This is evil - mass murder - in its trust form. How Angel can "excuse" these actions is beyond me and how Buffy can forgive him is shocking. To see her resort to a naive, trusting, wet-eyed, desperate school-girl in the presence of Angel is enough to make me feel physically ill.
Also Angel/Twilight does have a major valid point. In Chosen - Buffy & Willow did create an Army of Slayers - there by disrupting the natural order. With only 1 or 2 Slayers, they would have gone unnoticed on the Human Radar, and The Demon & Vampire communities would only consider them a minor threat.
Since when is it within Angel's right to play God? To kill hundreds of Innocent Slayers? This is what I have a hard time reconciling with Angel; his whole utilitarian attitude. If he saves the world, and wins his damned shanshu, he gets what he wants - but at what price? The death of the women he claims he loves and everyone she loves as well?
On the other hand - with 1,800 slayers - that's technical a small Army of Super-powered beings. Which could be considered to be a threat to the Human race & the Demon/Vampire communities. Couple with that the final act in "Chosen" was the complete destruction of Sunnydale - which the US government could consider to be an act greater than 9/11. With the U.S. Military(in a post-9/11 world) united with the demon & Vampire communities against a well-armed Slayer Army - I could also see how that could lead to an Armageddon-style Apocalyspe - that could end with lose of billions of human lives.
Since when did Buffy and her gang become the Bad Guys? If that's where the story is going - Angel needing to kill Buffy and her friends to save them from themselves - I'm through.
And yet also allowing him to pushing Buffy to her ultimate destiny.
I like to think that Buffy has the free will to control her own destiny. Angel does not define and/or control her actions. When they're together, he calls the shots, makes the decisions - with or without her consent. She trusts him and she shouldn't. It might get one of her friends killed.