No, it's not normal, and it's dangerous for you! He can bring STDs to you and ruin your health and your future. There are lots of guys who would never cheat -- never. Dump your loser and get a good man.
Biologically? Yes. The male human is designed to impregnate as many females as possible. However, we have progressed beyond this programming to control our reproductive urges and to enter into monogamous relationships with single female humans, therefore socially it is unacceptable for him to cheat.
"Normal" has a wide definition, and is usually defined by culture, morality, ethics, and laws. The key word you used was "cheat" - in some cultures, males are expected to have multiple partners - and they therefore don't consider this "cheating". In most modern, Western cultures, "infidelity" is frowned on - since people are often gravely hurt by such actions. In Islam, for instance, female infidelity within a marriage warrants the death penalty for the adulteress. This is not true for males, which are generally seen as superior beings than females. If you study science - different species have different propensities to cheat - some never cheat - bonding for life - others can't stop cheating any chance they get. Evolution, culture, social norms, penalties, etc. all contribute to whether you or your mate will cheat - often most of the choice to cheat is unconscious, as neuro-science has discovered more and more evidence to support this assertion.
No, never. It is not right to be with 2 girls at the same time! If they dont feel the same way about you as they did before than they should just break up with you but they should NEVER EVER cheat on a girl or anyone!