No sin inheres in the consumption of semen, so long as there was no sin in the emission of said semen.
It’s the emission that’ll get you: if you masturbated to get the semen, that’s a serious sin. If you were having legitimate intercourse, the semen should have been deposited in the proper vessel (your wife’s vagina), and intentionally failing to do that would also be a serious sin.
If, however, the emission were to be by accident – say, you have wet dream – eating it would be morally neutral.
One might argue that it is a sin of gluttony, because you are eating in such a manner that the sole object is your own pleasure, but semen is actually (albeit minimally) nutritious, and so the action is morally legitimate, regardless of its motivation.
A stronger argument would be that it may be a sin of lust, because, rather than attempting to satisfy the nutritive appetite (i.e. by eating things because they taste good), you would be attempting to satisfy the erotic appetite (i.e. by eating things because you get off on that). This would not be a proper exercise of the erotic appetite. Because it seeks sexual satisfaction “for itself, isolated from its procreative and unitive purposes,” it would be morally disordered. Lust, however, is not (in itself) a grave sin (although I suppose it can be under the right circumstances, given what Jesus said about adultery in your heart).
However, if you had a wet dream and (1) were starving to death in prison or the desert, or (2) were very curious because you’d never tasted it before, (3) thought it would make a nice garnish for the turkey dinner you were making, or (4) were super-excited because, all eroticism aside, you actually think semen tastes good, all these would be morally legitimate grounds for consuming your own semen.
Friggin’ weird, mind you, but morally legitimate. [EDIT: #2 is less weird. It’s not a rare curiosity, especially among teens.]
So, to answer your question: almost certainly not a mortal sin. Probably a venial sin against lust (because, let’s face it, most men who eat spooge do it because they get off on it). But possibly no sin at all.
DISCLAIMER: All of the above my opinion only. I am not a trained or licensed moral theologian.
DISCLAIMER II: In case you haven’t done this yet, please be aware that it does not taste very good, and, all questions of morality aside, not worth experimenting with just because of that.