I've never understood girls who consider "trapping." If somebody loves you, they'll stay with you. A man will move heaven and earth to be with you if he really wants to. Tricking him into having a baby will just cause hatred. And what makes people think that their partner will stay with them if they fall pregnant? They could still leave and you'll be left to raise a child on your own.
I had a friend who was having unprotected sex with this douche bag & I know a part of her believed that he'd stay with her if she fell pregnant. He'd always say things like "if you fall pregnant, we're moving in together and raising this baby together." They would have had a miserable life together. They fought like cat and dog, and he treated her like dirt.
It's such a huge risk to take and it's really not worth it. Especially if the other person does not want a child or another baby.