OK, perhaps this is an odd question, but here goes. My riding buddies and I have noticed over the last few years that when we (especially me) finish long rides, usually in the heat, and maybe (although we’ve not studied it that hard) when we haven’t had as much fluid as usual, that the veins on the side of our foreheads get bigger and stand out. What causes this? I have a theory (actually a SWAG) that these veins expend to allow more blood to cool your head/brain while exerting yourself in the heat. Is that anywhere close?
That sounds like a plausible reason. Could also be that exercise causes your blood pressure to raise.
http://www.scientificamerican.com/article.cfm?id=why-do-veins-pop-out-when
I think that is the answer. May not be specific to the veins on your head, but if not it’s an interesting read at the least.
Vasodilatation. Your blood needs to cool off somehow. Your body naturally takes your veins to the surface more where it is cooler. Take a look at your arms when you are warm verses when you are cold. If you are like most triathletes you may notice huge surface veins when you are boiling hot, but not so much when you are cold. I look at veins all day as part of my job so you tend to notice these things. Athletes and pregnant women are usually both easy iv sticks. I’m not a doctor, but I play one at work 
chris
That means you should always be dehydrated before having sex.