Uh, I'd go Ted and Grace. Despite what you all seem to think, she isn't exactly a seven year old girl. If you remember, back when tedd and grace shared a bed (one of the few times) she didn't seem at all angry or abashed at the idea of 'doing something naughty' but simply stated it wouldn't be happening due to no relationship (At that time in the comic.. that point is now invalid.) Add the fact that Grace is totally fine with being nude, or with others being nude around her. Hell, she even came up with 'Naked Fridays' remember?
The truth is, Grace is just easygoing and generally happy most of the time with a naitivtae about the outside world... buuuttt.. she did grow up with three brothers, and around other males. Secondly. I would say that if and when Tedd and Grace decide to copulate... Tedd would like them both to be human.. if you refer back to the time when he finds out that grace cannot become pregnant while in her squirrel girl form (no proof has been given that her other forms are immune to pregnancy ). Tedd says that it would mean more if she was human during the time.. but that it would be less dangerous if she wasn't. Ultimately, however, I would say that Tedd would leave it up to grace after giving her his views, and be happy with whatever she chose, Ted just strikes me that way... (No, I'm not calling him a submissive... Elliot.. however... J/k )
Elliot and Sarah strike me as the next to hit the haystack. Sarah seems to have a wildside indeed, and if you remember.. Elliot and Nanase made out at least a few times, so we know Elliot isn't exactly mister bashful (Even if he is a bad liar, and honest to a fault) .. even though Nanase was never physically attracted to him. Who knows how far they would have gone if she had been? I think that however, Sarah would have to feel totally certain about Elliot being there for her, and loving her totally.. before she would do that. She strikes me as the commitment type. And I'm sure that Elliot would want to make sure she wanted it.. and that she wasn't just giving herself to him because she felt she owed it or some such silly idea.
Ellen and Nanase come in Third Place, though I don't think Ellen would have any serious problems with the whole thing. If you've noticed she has exibited some very naughty thoughts. ( EG, the Video Store, and in Susan's house 'A Naughty Schoolgirl?') But Nanase has just recently admitted and found that she is a lesbian and thus I'm fairly sure that it will be quite awhile before Nanase is ready for such a thing. She's a strong, driven, willful girl. But she has insecurities that would have to be soothed.
Just my .02 cents