Hmm, I think that married stallions are not avaliable in the Alter of love. I mean,they are, when you're searching through your own ponies (on the left). I've just tried it - having written the name of the boy on the left side he appeared normally, but when I wrote his name on the right side he would not show up even when I marked what breed he was. So I guess it only works well when you choose your married mare as one of the couple (you know, place her in one of the two spots in the middle of the Alter of love) and the boy shows automatically as the only pony she can breed with :)
I hope this will help in the future, 'cause...twice through all my Celestial boys... OUCH - this is the most numerous group in my herd :P