You've got lots of responses of 'ew', but I'd rather look at it in a more practical sense.
For example, do you want kids? It may well be that by the time you're ready to start, he won't want any more. Or even if he does, his sperm won't be such good quality and you'll be at higher risk of having a child with a disability.
What about your career? When you're in your prime, he'll be thinking about retirement.
And most of all, of the women I know who'd married older men...the women outgrew the men intellectually. Thing is, for a 48yo man to be emotionally compatible with a 20yo girl, he must have a very young brain and approach to life. While this is attractive at your age, by the time you're 30, you will probably have outgrown him.
At any rate, if you want to give it a try - then do so. But don't marry him. Have a bit of fun with but keep the knowledge that it's unlikely to work out in the future.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.