Having unprotected sex with someone else, gay or straight, entails certain risks for disease transmission, so if you are swallowing semen, you may be exposing yourself to any disease that your sexual partner has (whether he knows about it or not), so it is important to talk about this with your partner, be tested for diseases in advance, and practice safe sex--which means generally not swallowing semen unless you have discussed safe sex with your partner and you both understand what this means and also trust each other 100%.
Otherwise, there is nothing about swallowing semen per se that is of concern.
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