Given how Lana and Mayson were portrayed in the series it would be unusual for Clark not to feel trapped in a marriage to either of them.
Lana's controlling nature and Mayson somewhat pathetic clingingness would quickly turn those marriages into unpleasant experiences whether there comes along a Lois or not.
If you hold true to Clark's basic personality as it has been presented in the continuity of the series; he would never cheat on either of them (or any other wife for that matter). But does that mean that he wouldn't seek a divorce to get out of a bad marriage? Would that make him a bad man?
Satistics show that more than half of marriages fail for various reasons. I would think that Clark would give his marriage more chances to succeed and survive than most would, but even a Superman can't create a loving relationship that isn't there. It does take two to tango.
In my opinion, Lois would become the catalyst for Clark to actually examine his marriage and realize that it's not what it should be.
If Clark is in a solid marriage with a woman he does love and loves him back, the addition of Lois would only introduce a wistful 'what if' element to his life. He would need a reason to end his marriage beyond a 'she's not Lois'.
Tank (who knows of a couple who got married to each other twice before they realized they weren't compatible in the long run)
