Ask yourself the following questions:
1. If she gets pregnant, what will you do? How will you know whether it's yours?
2. How many diseases can you possibly come into contact with by sleeping with someone who's sleeping with everyone else?
3. How low is Yourself esteem that you have allowed her to treat you like a fool?
I don't mean to be harsh, but unless you are both 16, you shouldn't even have to ask for opinions on this. My husband is only home for 1 week every month too. I would never consider sleeping with anyone else.