I think the implication of pon farr suggests that Vulcans don't have sex outside of pon farr, but it's vague and open to interpretation. I prefer to think that's not the case, and there's plenty of Trek lit that suggests otherwise, although off the top of my head, the examples I can think of are female Vulcans with alien lovers (T'Prynn, Selar) and those are with another woman and a hermaphrodite. Not that that means anything.
Regarding ability to have sex: I'm not sure where the information comes from for this book (I'm assuming it's gathered from novels, but I haven't read all the ones in question), but in the book "The Vulcan language" by Mark Gardner it says that Vulcan males are able to have sex at any time, the difference with pon farr being that they MUST do so at that time to avoid madness and death. Likewise, it says Vulcan females are fertile and able to mate all the time. Doesn't mention homosexuality.
Also says that, after the marriage ceremony, the matriarch presiding over the ceremony watches them consummate their marriage. 0_o
All right, now I've spent entirely too much time today contemplating the sex lives of Vulcans...