I just breezed through this thread but I'll throw in my 2 cents all the same. Personally, I think Angel's moment of true happiness has little to nothing to do with sex, least of all with climaxing. In fact we are shown quite clearly in 'Suprise' that it is post coital that Angel's soul is ripped from him. I think we have to ask why Angel lost his soul in the first place? What made him so blissfully content? The simple answer is Buffy and what she meant to him but more importantly what he meant to her.
Angel, both as a mortal and as an ensouled vampire had no reason to ever hope he could become someone of merit, someone righteous. A hundred years rehashing every act of cruelty committed at his hand had consumed him in utterly hopeless dispair. When you are that lost, when you hate yourself that much and have no reason to expect the next hundred, hell the next thousand years of your existence to be any different, how do you come out of it? What do you live for?
If you ask Angel, it is Buffy's name that you will be given in answer.
At the most basic level she gave him hope and a purpose. I don't know if you can really understand how truly meaningful that is until you have been lost yourself and had someone give you a reason to live again.
He fell in love with her, she gave his heart a reason to feel something more than sorrow. I don't for a second believe that Angel, in those early days at least, even entertained the thought that she would love him back and that is that is the crux of Angel's moment of true happiness. As unprepared as he was for his own feelings for Buffy he was shook to his core by her feelings for him. That changed his entire universe.
Of anyone, it was Buffy who shouldn't have loved him. She was supposed to look at him and see all the evil he had brought unto the world and recoil. Instead she looked at him with pure love, even when he wore his demon face. That must of shell shocked him, when he was at his most ugly, she didn't even notice. That's a kind of love that people dream about, how moving must that have been for Angel who lived in a world of nightmares.
I think that ultimately Buffy made Angel feel like a man and that was his undoing. When they made love in 'Surprise' it was a true expression of their love. They had been holding back in fear of their new and unknown feelings for each other. The joy in realising that the person that you adore, your reason for being feels the same way about you and you can finally express those feelings? No wonder he lost his soul. For the first time in his entire existence, human and vampire, Angel had reason to hope of heaven. Maybe he could make up for his past...if Buffy could love him, then anything was possible. He was a normal man, falling asleep in the arms of his normal girlfriend. It was perfect.
I don't see how he could have recreated that with Cordelia. He cared for her, deeply, but not on that level. She wasn't his entire universe, she wasn't his reason for living, she wasn't his hope. I don't think expressing his feelings for her physically would have touched his very soul. It just wasn't there.
As for the Buffy question, I'm in two minds with it. In season 3of Buffy I think he would have lost his soul with her because all the reasons that caused him to originally lose his soul were still there. Buffy loving him unconditionally and openly would still have moved him to perfect bliss.
Later on in their lives I'm not as sure, so much pain had befallen them both but I think that when they come together and can freely express their dept of feeling for each other it is perfect emotion. So, I'm going to take a leap of and say yes, that Angel could still lose himself in Buffy, simply because what they share is that strong and that meaningful.