I have never had a problem with sex during menstruation. The concept that there is something wrong with is just another instance of ignorance largely promoted by religious bigotry.
Indeed much of the pain experienced by women at this time can be due to the sense of rejection by their husbands for being "unclean".
If you want to seriously understand intimacy then try cunnilingus. All body fluids are derived from blood.
Poor joke really - if you spend that much on a lady for V-day and she's willing to stay the night with you, you'd expect to know her well enough to know when the time of the month might be... (or that she'd stay, she knows her schedule!)