As long as it's well-written, I don't have any hard or fast rules. If the characters seem real to me, if they make sense, I can follow them where they go
I had a reader bail out on a story of mine once because they couldn't stand to read Lois and Clark apart. I love them together,too. But I think if they are always together, no matter what, that grows stale.
I love the sadness and tension in Tank's Futures series, which has a great romance between Clark and altLois that ends in a 50 year marriage.
In fact, my favorite alternative couplings are the altLois, altClark combinations. I think there's a lot to be explored there, while still keeping a Lois with a Clark.
That said, it would take a really, really, really talented writer to help me go along with a Lois/Scardino pairing. As much as I like to keep an open mind, I just cannot imagine it!
But I did love one by SQD that had Lois married to Daniel (deep shudder) at one time. That was interesting, to say the least!
CC (tossing her two cents all over these boards tonight!)