Is It Healthy NOT to Ejaculate

Dear Dr. Betty,
When my girlfriend is giving me oral sex, she likes to place the pad of her thumb between my testicles and anus. If she feels me throbb and knows I'm about to cum, she'll push hard with her thumb there and squeeze the base of my penis with her other hand (index finger and thumb). She does this so I don't ejaculate into her mouth. Is it safe for her to do this to me? Where does the cum backwash go?
Dear JP,