Wow, I've struggled with this one too. I haven't gone on a vacation in 2-3 years but traveling alone would be a drag. I take the risk and wait for a girlfriend to appear in my life.
BUT, don't kid yourself, the time relaxed in romantic surroundings WILL lead you to let your guard down. You feel so strong now but when you can have all that time cozy and alone.....
I wouldn't take a third wheel either. (Do they have chastity belts anymore?)lol
No big wise advice pops in my head except this----If you two can go on a vacation together and are TOTALLY nonsexual while on it I doubt you are marriage material anyway. Just me.