Confused

A man wants to have lunch with me.  He is very nice and I do miss a man's company but I feel soooooo strange about this.  I know he would like it to go further than a lunch.
I was married to the most wonderful, considerate, supportive and kind man in the world for 30 years.  How can I be with someone else.  I feel I am betraying him.
I need help processing this . . . .