I have itch at the tip of my penis. I've had unprotected sex with my girlfriend a couple of times and this is the first time I am witnessing this. I changed boxers hoping the problem is dirt. So far I have no irritation or swellings.
Most likely you picked up a yeast infection. This needs to be treated as it is possible for the infection to travel up your urethra, leading to things like a bladder infection. However, you will need a doctor to make the determination.
Sex outside of marriage (fornication) is sinful in God's sight. "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4). Yes, I know everyone is doing it, but it doesn't make it right. I would like you to read: Why Sex Outside of Marriage is Wrong. One of the warnings God gives is that promiscuous sex will lead to diseases (wounds). "Whoever commits adultery with a woman lacks understanding; he who does so destroys his own soul. Wounds and dishonor he will get, and his reproach will not be wiped away" (Proverbs 6:32-33).
Treat your girlfriends with respect and don't use them for your sexual gratification.