How can I hide my sexuality from my parents (and do they already know by now)?

If you truly feel like hiding your sexuality from your parents is the right thing to do, then you can simply avoid making your sexual activities too apparent. All that you can do is, to instruct your sexual partner to not to show up too often at your place or else, that could certainly trigger a lot of doubts.

Furthermore, you can always hang out with female friends so as to prevent doubts. If you parents already have doubts about, they can certainly be removed. You can always get a fake girlfriend, or hangout more with female friends that are more accepting of your sexuality.