Spike and Angel (in text: "Angel and I were never intimate, apart from that one time.") Joss Whedon has also stated that Angel and Spike have had at least one sexual encounter, which technically makes this one canon. Ironically, in the Dark Horse Presents online special "Always Darkest", Buffy has a Spangel nightmare.
Angelus: Do you have any idea what it's like, having nothing but women as traveling companions, night in and night out? Don't mistake me. I do love the ladies. It's just lately... I've been wondering what it'd be like to share the slaughter of innocents with another man. Don't think that makes me some kind of a... deviant, hmm?