Well, if you don't want to get fucked, the most important thing is to NOT get caught - easier said than done, right? Try to kill everything from a distance as best as you can, and never run out of ammo, as you'll be stuck unable to move during the forced reload animation.
Having panties on can help, as it prevents penetration, but it won't last very long. If you want to keep virginity, having a few panties in your item stock can help: don't break free until panties break, so you can put a new one on immediately.
Once penetrated, watch your lust gauge and use the love potion to prevent orgasm. The willpower potion restores your HP = max struggle power, so use accordingly.