I’m not sure about this.
I am married, but back when I was dating, I probably would have dated an SSPX girl. I did date some protestant girls, but I never even considered marrying one of them unless she converted beforehand.
But SSPX? Their baptisms are valid. Their doctines and beliefs are in no way different from ours. Their orders are valid. They perform the Tridentine Mass, which is valid. They don’t deny the primacy of the Pope.
As near as I can tell, the only way in which they are not in union with the Vatican is that they reject some of the documents of Vatican II. I think one on ecumenism is one of them. Not sure what the others are.
But I would have reservations even so. From what I know of SSPX people around here, not many would attend the N.O., even for the sake of family unity. But if the girl would??? I don’t think, in that event, that I could rightly reject her just because she’s SSPX.
I should add that SSPX ers are not all the same. One of my granddaughters, in fact, has dated an SSPX boy. He has attended N.O. Mass with her, but has never asked her to attend an SSPX Mass. If one asks him about his religion, his response is: “I’m just Catholic.” I’m not sure whether there is a difference between some of the older generation and the younger ones, but I suspect there is.