Saliva will only prevent pregnancy if you engage in oral sex instead of ejaculating in a fertile vagina. It doesn’t matter which of you is giving or receiving - just keep your genitals apart.
Vaginal sex without contraception causes pregnancy. Saliva will not stop those sperm swimming to target.
If you don’t want to be pregnant, or make someone else pregnant, then you need to use a condom every single time you have sex. She also needs to use a hormone based contraceptive such as the pill, implant, injection or a coil. Doubling up on methods gives you maximum protection from pregnancy and sexually transmitted diseases.
I’ll go further. If you engage in oral sex, in all probability you will eventually have penetrative sex. The human race has survived and flourished because the sexual urge is strong. You’re less likely to get pregnant or diseased if you have full sex with two methods of contraception as I’ve described than if you trust to abstinence or luck. So you might as well enjoy yourselves - safely.
Be prepared, get contraceptives and use them.
Condom for him. Hormonal contraceptive for her.