Well, if in the future we all are gonna end up in a nursing home, isn't it better to have someone there who shared a life with us instead of being completely alone? I mean I don't think we have to take care of them in a nursing home anyway.
Also, most people don't like to take care of an elderly when they are still young and can enjoy life. You seem like you are ok with it, which is fine. But most people seek energy and youth as they get older and I think that's one reason for why many men and women like to hook up with younger people, it makes them feel youthful again. Now imagine if instead they had to worry about things a much older person would worry about, obviously it would make them feel older and most people don't like such a feeling.