Recently my husband and I just bought a mattress.

It's nice of you not to mind splitting, but I think you should still have a good discussion with him as to what both of you think of your current expense distribution and what is considered fair. Diff people have diff interpretations of fairness, you may be thinking you are accomodating him, while he may be thinking that he is being more than fair. This should be brought up to the open now since this is the first time the topic came up, in case this becomes a festering issue in your relationship over time. I don't think the immediate reaction should be a tit for tat and ask him to pay for other items. He's an adult and I doubt his parents can influence him that much. Does he think he had little say over the mattress purchase and is angry over it? Is he unbalanced because he felt he had already been paying too much in the past? Is he just trying to placate his parents and intend to make it up to you next purchase? Before having a discussion let's not jump into a potential quarrel about past purchases. You're in this journey together, you will start off with different expectations, but mutual understanding, not judging each other, and open communication can bring you together as partners.