Yeast infections occur when there's an overgrowth of yeast that inhabits the vagina normally. Yeast isn't transmitted from men to women.
A healthy vagina is slightly acidic, measured by pH level. A pH level of 4.0 to 4.5 is healthiest for women; menopause sometimes affects pH because the loss of estrogen reduces circulation and lubrication. Semen has a higher pH level, which means it makes the vagina briefly and temporarily more alkaline and more hospitable to bacteria.
Condoms will prevent that brief pH disruption, which may be helpful for some women. Others use RepHresh, a pH balancing product, to prevent bacterial infections after intercourse, but note that only condoms provide an effective barrier against other sexually transmitted infections.