Go wank it off in the bathroom.

Well, doing it here would be kind of weird. Plus someone would have to come and clean it up if you got cum on your bed. So, just to be safe, you head over to your personal bathroom, lock the door, and unbuckle your belt. Without waiting any further, you begin jerking off, doing it facing the toilet so that any bits of cum you shoot end up going in the toilet bowl. This way, at least nobody would know you had been relieving yourself constantly like this. Panting and grunting loudly, you push a hand to prop up against the wall as your hand starts speeding up, stroking and beating your meat harder and harder.

You can feel it stiffen, hardening and twitching, as some lewd thoughts come to your mind. What if you had Artoria Lancer give you a titjob? What if Boudica and Raikou mommied you, each one feeding your mouth with a tit each? It'd certainly feel nice to feel up Okita's thighs. Having Musashi's lips around your dick would feel so good. And Mash? Ooh... impregnating your Kouhai would be quite wholesome...

Gnh! Here it comes!

Your cock begins to spurt and shoot strings of cum into the toilet. As you do so, you hear a voice calling out for you, before stepping into your room. Oh shit! Good thing you at least did it in here. Quickly, you clean up, as you flush the toilet, erasing any trace of what you did, before washing your hands and stepping out. "I'm here, just finishing up using the bathroom. Oh, hey Medb!" You call out, as the Queen of Connacht stands in your room. Seems like she was bored and wanted some fun.

Phew. At the very least, she didn't catch you doing that. No need for the rest of Chaldea to know your cock size or know how sexually frustrated you are. It's probably safer to keep doing it this way.