But had you been married to the maharaj types in the first 3 yugas, maybe you could have been married to Vishnu. Now what would it be - marriage to Vishnu and participating in an Ashwamedha, or lose the Ashwamedha but give up getting Vishnu as well? Hmmmmm????
I recall Chen2 explaining that the only reason that KSK were involved in Dasharath's yagna was that it was a Putra-kshameshthi yagna, unlike Rama's yagna, which among its other goals, was to atone for Ravan's brahm-hatya. Therefore, from a poetic standpoint, Sita, who was the cause of Ravan's end, couldn't possibly be there. I don't think it would have been an issue for subsequent yagnas, if, say, the Ananda Ramayan was correct and Sita was re-united w/ Rama after her underground break.