Probably nothing, but there could be nasty side effects...
It depends on how much action they see and how supervised they are on the battlefield, and what they do with their shore leave.
If your men are trained, drilled, and then sit on bases or expensive tech doing not much, then getting 2 trips home a year or something, absolutely nothing will change. They will write letters to their high school sweet hearts, and those relationships may work or they may not, but it'll be treated like a pause on all relationships between ages, say 17 to 20.
(Could arguably even be a good thing to force a pause on all young relationships between teens and adulthood to stop regretful marriages - We have cooling off periods with major financial contracts, relationships should be no different).
However if they see action, especially deployed into enemy territory, and they're not very well supervised or held to account for their actions, this could have major impact on your society: Young men, sexually frustrated and inexperienced, sweeping through enemy towns with automatic weapons (and the ability to kill anyone without consequences) have a nasty tendency to rape, both historically, and in modern wars. Even those who don't commit the acts directly learn from the culture - having buddies do these horrible things normalises it.
Even minus the rape of enemies wife's and daughters, sex for an army is often at best a transaction at a local brothel - teaching a transactional model to sex "I gave you money, now we have sex" that often doesn't fair well when equal adults attempt courtship. "I bought you dinner and 2 drinks - now you owe me sex!" - "But I have a headache" - "You're trying to rip me off!".
While I doubt they'll bring the serial rape home with them (it'll probably get compartmentalised - "the enemy is subhuman" "Laws don't apply in that land, but do at home"), their deployment was probably their first few sexual experiences and the bulk of their informal sex-ed. If a significant number of your adult males return from the field with that learned experience of interacting with women, there will be problems. Your males will struggle significantly with concepts like informed consent and dealing with rejection.
If they learn their sex-ed from a position of power - I would expect issues with increased domestic violence in your society.