He's too big!
It doesn't matter which side of the fence you sit on when it comes to big penises, initial vaginal penetration can be painful for any women if she isn't physically prepared for it.
Your vagina should be able to accommodate any size penis, but it must have time to do so. Maximum foreplay before sex is vital to preparing your vagina before having sex with a well-endowed man. Your vagina lengthens and 'barrels out' the more aroused you get, with your cervix and womb tilting upwards and backwards.
Preparing your vagina so that it's aroused enough to accommodate your man's big penis can take up to 25 minutes, otherwise it could be extremely painful for him to enter you too soon.
So, think plenty of foreplay and tons of lube (check out Good Vibrations' Good Lubrications Cream and Aquaglide Original Lubricant) before attempting penetrative sex. You'll find that as you get more aroused, you'll continue to expand inside, making sex easier for you to take with every thrust of your lover's big penis.