I don't know about the legality of it, but I think nowadays being roommates isn't that rare to share costs. The idea of two opposite sex couples sharing a house is less likely to spur questions that if it was one man and three women or one woman and three men.
I understand the worry but I think if you're natural about it, people should leave you alone. If asked, just say you're friends and figured living together would help you because there would be more adults around all the kids.
I wouldn't recommend being open about it, but that's really the only advice I can give you. I hope that someone will have better law-oriented advice for you.