Why don't you just ask your wife if it is ok if you hook up with her just one time? Your wife might have some hot guy whose crotch she keeps looking at and might be agreeable to letting you screw her friend if she gets to screw someone else. I assume that would be ok with you if she did, right?
My bet is that this is simply a matter of you imagining she is staring at your crotch because you would like to think she is doing so. There is no real indication that she would be interested in ruining her friendship with your wife simply to sample your wares.