I wouldn't find it nearly so strange if they didn't seem completely sincere. I probably wouldn't even notice if it happened while I was walking outside, down the street for example.
Is there something about the fluorescents in the lobby that make me look particularly marriageable? Or perhaps particularly desperate? The closest thing to a sensible explanation I can cobble together is these men are perhaps late from INS, and thus find my citizenship desirable. But that requires that they would have to 1) notice my badge 2) identify that my employer is indeed the federal government and 3) be aware that US citizenship is a requirement for federal employ.
In demeanor they have struck me as someone who is about to ask for directions to the cafeteria, or whether they are at the right bank of elevators to reach a certain floor. Both times I've felt vaguely unhelpful, more than anything, in my refusal - and both times my negative response has been greeted with "just thought I'd ask." Did I miss the notice saying it was National "Ask A Random Stranger To Wed" Month? If I said yes, would they laugh and say 'just kidding' or would they expect me to march up to the county courthouse then and there?
It's just a little strange to me, I guess.