That wasn't physical love. Tifa was emotional about not having anyone to go back to, unlike everyone else, while Cloud was brooding over the loss of Aerith. I feel sure they had sex, but it was not out of love for each other. It's a classic case of two people who had lost something dear to them and took comfort in each others company for relief. At the end of Advent Children, it is possible Cloud finally decides to move on with his life. With or without Tifa is up to the imagination.