The same reasons you feel comfortable for her exploring sex with a woman is true for exploring sex with a man. PEOPLE ARE NOT INTERCHANGEABLE! You seem to be under the misguided notion that because you and other men share the same type of plumbing that it is just a matter of your wife finding a guy with plumbing she likes better, etc. I can guarantee you that she married you for 1000 other reasons than your plumbing.
Now, the question I would ask, is do you and your wife have real issues in your marriage? Issues that could be exacerbated by adding new lovers. In regard to you adding a lover, it sounds like your marriage is very strong. But the only true danger I see, is if in fact you two have issues that haven't been fully resolved (outside the general insecurities many have when embarking on poly).