There's a lot of confusion about co-signing a loan and it's not uncommon for someone to think that they can co-sign a loan... only to be surprised later when their credit score drops dramatically because the other person in the relationship didn't pay.
I hear about clients who are engaged and buying homes together because the interest rates are so low and I cringe and pray that it's all going to work out perfectly in the end. My daddy always said, "Don't buy anything with anybody you're not married to."
More marriages have problems due to finances than infidelity. You need to remember that you are partners in more than just love. You are business partners also and the financial aspect of your partnership needs be established.