I was raised heavily Christian, so to answer that you've got to be aware there are all sorts of sects, and every single one of them selectively picks and chooses particular interpretations of things, and focuses/ignores parts of the entire bible.

According to the Catholic Church, all sins are sins. They're just bad. There's really no scale, per se, but the gravity of it does factor in on how "bad" it is.

Marriage is defined as being for a man and a woman, and it's FOR procreation, not love, although love is a plus and what holds people together. So, a divorce is bad, too, since it's not even allowed. You can get an annulment, which is an official dispensation that it was on false grounds, but that's still a sin.

Given the modern frequency of it, I think a gay marriage would be seen as worse, but getting married "outside the church" is also a sin. So, you'd be having two sins in that case. And, then you'd be having homosexual activity, another sin, and each time it's done after, as a sin.

As for a Buddhist marrying a Christian, it's allowed, but needs to be in a Catholic Church and go through the whole Pre-Cana process. It's a considered a sin for the Buddhist to not convert, and not go through all the sacraments, so tack to hose on. Child-free atheists are sinners in countless ways by not being part of the church, not believing in God, it would take forever to go over it. They'd be considered unmarried in the Church and having pre-marital sex, even if it was done by the state. Their marriage could also be considered a sin because it was not done for procreation.

The rules are kinda funky and rigid, so that's why I say "raised" on it. Too many disagreements, the whole thing is on "pause" for the moment for me.