What do you think about me using my gf's allergy to my advantage?
How are you going to use it advantageously?
My gf thinks I care about her and that's why I never tried to cheat on her, but I don't.
Taking advantage of your girlfriend is wrong. Some allergies are uncontrollable and you should be helping her out with them.
That's her problem. Her allergies will provide me a plausible excuse to cheat.
The plan you're concocting is devious, disrespectful and cruel. If you don't love her and want to date other women, you should summon up the courage and be direct.