Don't be an idiot! Your wife will be fine, it's you child that will change your life, including your sex life. You will no longer be number one, you will be last on the priortiy list. Take good care of your wife and child, it will be worth it.
just go down and do it. Once you do it a few times you'll love it. Tell your gf about your fantasy. I did, once day after I'd come she asked me to go down, had a hard on for three days just thinking about it.