Is my boyfriend marrying me out of convenience?

My boyfriend is planning to propose soon and I know this because we’ve talked about it, but whenever I ask why he loves me and why he wants to marry me, the answer is often “i don’t know, cause I want to” or “why not?” He’s also answered “so it’s done” implying that so we wouldn’t have to worry about it anymore. These answers make me sad as I don’t feel sincerity in his feelings. There were also times when I would bring up getting engaged, our wedding, living together, or kids and he doesn’t respond a whole lot and is poker faced like he couldn’t care less. But then there are times when he would get into the conversation and go along with me. He’s expressed that he’s at the stage in life where he wants to settle down and start a family but I can’t help but think what if the only reason were getting married is for him to get that? I just feel confused as he’s hot and cold with me. Would you consider this a red flag? Am I just overthinking this?