I don't thing you're gay in the slightest. Alot of guys find themselves looking at penises (in the locker room, the internet, etc.) and wondering if they are gay. They're not. Most are not even curious. The only reason why they do this is because they are worried. You said yourself (by asking) that you don't know if your penis is big or not; so most likely you just want to compare with other guys to make sure you're big enough (and you are
As for feeling aroused, that's simple. Looking at anything sexual for any amount of time can get our hormones pumping; so the straightest guys may feel... "uncomfortable".
You said you like girls. Not guys. I explained why you felt aroused. Your most likely not even curious (and definitely not bi). You are straight. You just worry about your size, that's all. YAY!!!
BOTTOM BOTTOM LINE
Just so you don't feel weird, alot of guys check out other guy's "junk". And they aren't gay either. They just want to see how they "measure up". Heck I'll even admit I've snuck a peek when I was feeling "inadequate" in the southern department!
I hope this helps... quell your doubts...