It's not wrong. It's totally possible for two persons of those ages to fall in love. However, the chances that it will turn into a permanent relationship are pretty slim, in my estimation, though it may last a few years. Regardless of the genders of the two persons, these two individuals are unlikely to be on the same page for more that a limited span of time. The 45 year old will be starting to slow down with age, just as the 20 year old is coming into his prime in terms of strength and energy. Their interests will diverge. They'll tend to find that others closer to their own age are more companionable. It may be a valuable experience for both of them, but I would call it a "stepping stone" affair, from which they springboard off and move on to something else. One of them may end up quite hurt. Then, again, that can be the outcome of any relationship.