There is nothing in the Scriptures that prohibits a man from swallowing his own semen if he so chooses. In fact, this is an ideal solution for avoiding the sin of “spilling seed” if the female partner is not willing to swallow, or if the man is masturbating alone. As you point out, there is nothing homosexual about swallowing your own semen because it does not involve having sex with another man. However, if you do choose to swallow your own ejaculate, be careful not to get any on yourself or your clothes. The same requirements regarding semen and cleanliness still apply (Leviticus 15:16-17).