Your sig refers to your journey.
Somewhere on this site is a thread about the theory of abundance as opposed to scarcity. There's enough love to go around. Love is infinite. The more you give, the more you get back. In your case, the more love you 'allow" your wife to give to this partner, the more you will get back. She will be more fulfilled, and a better, happier partner for you.
This takes trust of course, and a firm foundation in your relationship with her.
Lets see, what is the worse that could happen? She loves him more than you, and leaves you. Sex alone wouldnt cause this, she could fall in love with anyone else at any time anyway. This usually happens when people grow apart and goals and interests change tho, not just because they found someone better.
Now, sure, them having sex will increase their bond. He could become her secondary, and there is their NRE to deal with. But she won't leave you for him, if she really does love you. Her love for you could in fact increase, with her gratitude for your generosity in sharing her w her new lover.
In time, and with your participation, he might become a 2nd primary for her. but here is where you have some power. If you're not comfortable with that, you can work to set boundaries around how often she sees him, calls or texts him, whether she has overnights or vacations with him, etc. IF you really like the guy, and he and she continue to be in love, he could eventually move in with you as part of your family, adding a richness to your life.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve.
~Percy Byshe Shelley