For me personally (because you asked "what would you do")
I would stop having sex with any partner who didn't sit down and agree to what our SPECIFIC terms for sex with others is BEFORE having sex with someone else AND keep those agreements.
Agreements can always be re-negotiated, but until they have been, they are the agreement.
Trust in a relationship is critical to me.
IF my partner wanted to take risks I wasn't ok with (and risking pregnancy is one I am not ok with as I don't want to raise any more children); I would stop having sex with them and if a pregnancy arose, I would terminate our relationship.
**it's quite possible I would terminate the relationship over the not using protection in the first place. But I would DEFINITELY terminate the relationship as the point where a pregnancy arose**
"Love As Thou Wilt"