this is my fall avatar
Well...yes and no.
In the Irish/Celtic Catholic community, celibacy is supposedly only asked the clergy, though abstinence from masturbation and premarital sex are discouraged for all. So, yes in that they'd be celibate now, but no in that it would be a no-brainer just because they're them.
But beyond that fact I do agree that adding a romantic plotline would cheapen the films -- honestly it'd be the nail in the trilogy's coffin, because that's not what it's about and employing that kind of B-plot to give the films direction is just severe disengagement from the plot, as well as a big middle finger into the face of the 'premarital sex' views of the Catholic faith. Besides, I view them more or less asexual, so I don't think either of their marital status' will ever change anyway.
TL;DR yes, but only because they're not married (and I doubt they ever will be)
I did read about the premarital sex thing in the Irish Catholic faith, so I guess that could be the reason, but then again, two famous homicidal vigilante's on a holy mission don't strike me as the sorts to go getting married.
Saying it'd be a no-brainer just because they're them (i assume you mean because they're young men who drink and socialize?) kinda doesn't fit either for me, because as you mentioned, if not celibate, the next assumption would be that they're asexual. The depiction of the their characters lifestyles/habitat/personalities, even before they became the 'saints' didn't give me the impression they were looking for a potential girlfriend or wife.
But yeah definitely agreed on the hope that the third movie (which has apparently finally been pitched to a studio) will not have any kind of sexual plot line, it just wont do the original story idea justice, but I have faith in Troy Duffy, he didn't do it in Boondock 2 despite the female character and his so far revealed plot idea for 3 has the 'saints' in prison, so I think it's unlikely.