was one of them married before and the spouse died? because there is a game bug that leaves them flagged as married (or engaged if they were engaged instead).
if this is the case then there is now an easy way to fix them, JM Pescado at MATY just recently updated his Lot Debugger hack (you can use it to fix many game issues, like when a car goes missing because the owner is back on the lot after a reset from installing certain hacks or an EP)
http://www.moreawesomethanyou.com/smf/index.php/topic,72.0.html