I don't mind alternate pairings IF:
1- The character is someone I like. (Meaning the author presents him/her in a sympathetic manner, and I can see Lois or Clark falling for this person.)
2 - as mentioned by others, no adultery, cheating, etc.
3 - Clark can be with someone other than Lois AS LONG AS it is someone he trusts enough to tell his secret to.
4 - The relationship must be genuine. Which also means I'm not automatically hoping it will end in a break-up just so Lois & Clark can get together in the end. (I'm old-fashioned here. I don't believe in falling into and out-of love.) But even on this point, I'm willing to go where the author leads me, as long as the story is well-written.
Actually, I'll qualify #4. It has to be genuine on the part of Lois or Clark. If the other party is just stringing them along, and they eventually find out and break up, then end up with their one true love, well, that's a different story altogether!
- Vicki