Dear Boner Boy:
Don't think of it as an inappropriate erection; think of it as an opportunity to advertise.
Odds are, you aren't going to marry your current girlfriend, so why not let her hot friends know that you're ready for some serious fucking at a moment's notice? If you're lucky, one of them is an unconscionable whore who'll attack you while your girlfriend is in the shower tending to her sandy vagina.
This conversation or a variant thereof should happen at least twice:
Girlfriend's hot, bikini-clad companion: "Ohmygod, do you have a hard-on?"
You: "Why yes. Yes I do. Gaze upon it in all its glory!"
Even if none of these ladies are impressed enough by your renob to betray their friend, I seriously doubt any of them will be offended. After all, they aren't wearing bikinis to make your junk recede into your body cavity like a turtle's head. And if your girlfriend calls you out, tell her she's the one who got you all excited. Thanks to our old pal Cognitive Dissonance, she'll believe you.