I would say that it is safe only if you wear a condom if you guys are in a committed relationship you should go get tested for STDs so that the baby doesn't get anything from the both of you. You should be wearing a condom anyway bc of STDs and she doesn't kow who you are scr*** and you don't know who she is scr***. If she is pregnant and no longer with the baby's father and you are trying to step up to the plate remember its a great responsibility and that the mother and child's health are most important not that it feels different. If you are just getting your rocks off stay protected.
why would you have sex with a woman who is pregnant with another mans child?