Buddhist priests have to be celibate after giving their vows but they can remove their vows at any time, only to take them again later.

Souichirou doesn't seem all that serious about it but Issei's entire characterization point is how serious he takes his religion. He's obviously planning to reach enlightenment and to do that, buddhists need to remove all physical things that hold them back. I doubt Issei wants to have anything to tie him down to his physical body and distract him which is why I seriously doubt he wants to be involved in any relationship until he can reach the status he wants to be at.