If my boyfriend was rubbing his penis on my butt (in my butt crack) and facing downward for about a minute, is it likely that precum could cause preg?

Possible, unlikely: It would be very difficult to explain conception by that mechanism, but it is technically possible. If you're concerned, take a pregnancy test and see your physician. Safe sex practices, such as condoms, can help prevent the transfer of STD's, as well as undesired pregnancies. There are some STD's that can be transmitted via anal intercourse.

Vagina: No. Your anus is not the same thing as your vagina. Sperm have to be deposited inside the vagina during ovulation to cause pregnancy.