hmm.. odd. Well you might need to know that FBSD is all xorg now (legacy still uses xfree though). So.. what display manager are you using? IMO, this is the first problem, display managers are annoying but it's your choice. Moving along, try to see if X will boot - just X. No display managers. I dunno why it would boot you back into the login prompt unless the password wasn't correct or if you were trying to login as root and do not have wheel permissions. Do
I believe and that will create a base xorg file for you. Now try running X with the startx command. If you really need a display manager you might want to Google the problem a bit or try editing your .xsession by hand.