What is the reason you and your fiance are trying to have a baby outside marriage? I agree that both of you need to be tested for fertility.
If your test shows that you have too few sperm to fertilise her eggs, would you be willing to let her sleep with horny men to give her some babies rather than going through the stress and cost of artificial insemination.
It would be reasonable for your wife to carry on a sexual friendship with any man who got her pregnant, as afterall, he would be the father of the child, and ideally would need to visit often to spend time with his child and to make your wife pregnant again and again until she decided she had enough children.
You could restrict your wife to just having sex with a horny man for her fertile days, but this would not be as satisfying as having ongoing sex with him during the pregnancy, so as to develop a strong bondage attachment as the birth drew closer.
Ideally, you and the father-to-be should both go to anti-natal classes with your wife to support her adequately and show you approve of the arrangement.
It can also be helpful if you and her lover attend the birth together, so you can immediately thank him for making your wife happy and adding value to your marriage. This can be a powerful bonding time for all three adults and establish what happens from here on.