Speaking of marriage
I attended a wedding on Saturday, the first Byzantine wedding I've ever seen. Unfortunately, I wasn't able to pay much attention since I was busy trying to scrape a presentable tenor part out of my strained vocal cords. But at the homily, my pastor gave them this advice: "If you want a long, happy marriage, pray together. Start it tonight, before you `retire'." I think that is quite right.
Think about it: you say grace before you eat, don't you? Food is a gift from God, and one ought to thank him every time one eats. Sex is also a gift from God. I think saying grace before sex is an absolutely splendid idea. You could even use the same prayer: "Bless us O Lord, and these thy gifts, which we are about to receive from thy bounty, through Christ our Lord. Amen."
P.S. If praying before sex spoils the mood, may I suggest that you are probably doing stuff you shouldn't be doing?