Ehhhhh it can go both ways, but typically I would be a little uneasy if my girlfriend/wife had a straight male best friend. I would say there's an 80% chance the friend would date/sleep with you if you said yes, 20% chance it's truly just platonic.

I think if you weren't in a relationship it's a no brainer that male best friends are fine, as long as you know there's a better chance than not that if you ever gave them the go-ahead they would take that in a second.