Yeah weird, I think everyone has their own reasons, but I agree that women get labelled a slut or similar if they show how interested they are in sex.
When I have asked a few guys they say they find it 'rude' and/or 'inappropriate'. (LOL Get over yourself).
Asking/Lying is just a big game really, I mean, don't lie, but be diplomatic and I think the person would respect your honesty more and it hold greater weight with them.
*Hums Lilly Allens song 'The Fear' in her head*