Really? .. Really?
As a woman who has sex as often as possible and also bikes as often as possible (Brooks saddle, minimum 14 mile a day commute even without extracurricular events), I've had my share of woes, but certainly not that regularly. I hate to say it, but my initial reaction is 'Dude, ur doin it rong.' But I'm sure that if she is enjoying it at the time, that's probably not the case. I'd give a bunch of different preventative advice here. The petroleum jelly wasn't a bad idea, but of course that's not the product to use. You want something like
Wet , they have a huge array of different lubes perfect for whatever your needs are, depending on if she's sensitive to certain things, if you use condoms or if you're regularly silicone toys. Also, she'll want to hose off the love juices immediately afterwards, because since your fluids have a different PH value than hers, prolonged exposure can cause irritation and swelling. I recommend bringing baby wipes. Last but not least, the crappy thing about prolonged riding plus the vagina is that it can create the perfect stuffy, hot, moist environment for yeast to flourish. This may not be noticeable regularly, but get a little friction up in there and boy howdy will you feel their presence. I recommend yeastgard, available at any CVS - it's a homeopathic product designed to balance out the internal ecosystem by introducing more of the yeast predators to the area - just pop one in after sex or on any night she's feeling a bit rough, and everything will be fine in the jungle in the morning.
All of this is tertiary to correct posture and having the right seat, but you guys know that stuff already. I am absolutely positive that cycling and sex are not exclusive pastimes. Please let us know how it turns out!