I would be uncomfortable with my partner hooking up with a stranger. That’s a betrayal of our shared values, and creates risks for both of us (e.g. unwanted pregnancy, STDs, and general drama that could create a lot of unnecessary stress). I would rather know that she has fallen for someone we’re both know preferably, and who she loves and believes she can trust implicitly. I would still feel a healthy amount of jealousy, and be worried about thing going wrong, and her being hurt. But I would trust her judgment, and trust she won’t run away and leave me to raise our kid on my own. But yeah, the sex, and intimacy in this case, wouldn’t bother me too much.