In this day and age, it seems that the "till death do us part" does not hold up anymore. If a serious relationship nowadays lasts 20 years, that is pretty good.
I have a different view on serious relationships than I did when I was in my 20's. Make the best of your current relationship because you never know how long it will last. With that said, I don't see a problem with older men getting involved with a younger woman.
Even if a relationship was to only last for 5 years, as long as that relationship was a very happy one, why not? If her and I could be very happy and in love during those 5 years, that is a lot better than not being happy and in love.
It does not matter if the woman is 50 or 30 years old. When you meet "that" person, both you and her will know it. That is where all serious relationships begin regardless of age difference.