Well you ARE betraying your best friend then. Sounds a lot like this isn't the wife's first time propositioning someone other than her husband though. Tell her what you're telling us, that you feel like screwing around with her would put you on a massive guilt trip that you don't think you can take.
And a word of advice, stop thinking with your dick. That getting laid excuse is fine for a college student but you're a grown man. If you really want to think objectively then pay a hooker for sex, blow your load, then reassess whether your best friend's tits are worth it.
