Despite how enlightened and intelligent the Vulcans are, they have very Victorian attitudes towards sex, so it is not inconceivable that there would be some more "traditionalist" elements that would looked down upon Vulcans who engaged in: sex with aliens, sex outside of pon farr, sex with the same gender. That would've made a little more sense than for some unexplained reason Selar's gay brother is immune to his impulses. Just a thought.
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...
D.C. Fontana's interpretation has always been that Vulcans are capable of sex at any time, and pon farr is just the time when the drive overwhelms them and it becomes a matter of compulsion rather than choice. The Trip-T'Pol relationship in ENT would seem to prove this canonically. And "All Our Yesterdays" also supports it.
Wow, that is bizarre about the matriarch watching the marriage being consummated. I agree that Vulcans can, and clearly some do, have sex outside of pon farr. Who am I to disagree with D.C. Fontana? For what it's worth, Selar's brother was pretty emotional for a Vulcan. He seemed like the most well adjusted Vulcan I remember "seeing", which could be an alternate explanation for why he didn't go thru pon farr. I'm treating that as an individual situation that he specifically doesn't go thru pon farr instead of the wider suggestion that all gay Vulcans, or at least the males, don't go thru pon farr.
Something like this was actually done in medieval Europe, at least with kings, since the consummation of marriage was important for determining inheritance and the legality of marriage.
I remember reading, I think it was France that used to have an audience watch the marriage consummated. Other countries may have done it too. I know that the French queens gave birth with an audience to make sure that an infertile queen didn't sneak a fostered child into the line to the throne.