This will be interesting to everyone who comes here.
Redbook did a male/ female sex survey that has some surprising answers.
Rather than copy it here, do a google search on "Redbook Sex Survey".
It's perfectly normal to fantasize about it. No fantasy is a bad or no normal fantasy.
If you are thinking you might want to go beyond fantasy - go slowly. First talk about your fantasy as a dream. Ask her what she thinks about your dream. Depending on what she says- you know whether to keep it a fantasy or talk about it more.
You may find that just talking about fantasy is enough. Or pretnding to do the fantasy.
Some people take the fantasy to reality - but b absolutly sure that is what you want. Sometimes it chages relationships in good ways,and others in bad ways. Sometimes th fantasy is better than the reality. Sometisyou dcided you do;t lik it- but he does. Be cautious.