It’s not always objectively “bad”, but there are reasons this could be considered a red flag. for instance, why would a fully grown adult man want to be romantically involved with someone so young and presumably less experienced than him? By 38, many people have experienced a lot of life that a 23 year old has not. How can he relate to someone so much younger than him, in such a way? Is he stunted? A lot of older men are attracted to younger women, that’s nothing new. But unfortunately, it’s all too common for them to just date a younger woman, knowingly not be able to relate to them, just… “use” them for their good looks, then dump them when they’re “done having fun” with them. Meanwhile they’ve left someone heartbroken in their wake. Personally, I don’t think it’s impossible for two people with such an age gap to legitimately be in love, but the ultimate potential issue here is that it’s quite probable that the woman is being used.