I couldn't find anything on this, but that doesn't mean it isn't causing increased outbreaks for YOU. It could also be that you are having more and/or longer sex, and the friction is causing more outbreaks. Make sure there is enough lubrication. Even if your partners are female, extra lubrication may be needed.
You don't mention if you are being prescribed the Viagra, and I know that some men take it themselves to last longer, and not because they have ED. If you fall into this category, try not taking it and seeing if it stops the outbreak frequency. (If you fall into this category, you might want to consider talking to your doctor about taking it, since it can cause cardiac side effects. Make sure it's safe for you to take.)
Are you on a suppressive medication, like valtrex or acyclovir? You might try that and see if it helps.