English novel, probably published in USA.
The book starts off with a man, the protagonist, (aka Hero) leaves his village/tribe(?) and journeys to a remote area where he meets up with a woman who has traveled there from her village/tribe. IIRC both wearing animal skin clothes and armed with bows/clubs/knives only. They wait (a couple days?) there and while the woman is willing to be "friendly" the man prefers to remain chaste. (that's what his tribe believes is required) Eventually they are picked up by a spaceship, where they are separated. The Hero joins a bunch of other men picked up from other tribes (all picked up as a couple, and many of the men comment about getting "friendly" with the women while waiting. Only the Hero appears not to have done anything.)
It appears this planet is used as a breeding ground for the aliens who come once a year to pick up new recruits for their war.
The men go through training and get issued an advance skin tight full body suit/armor (with AI? or maybe just very advanced interface.) The suit is more like a scuba wetsuit in that it is flexible. IIRC the Hero always takes off his suit when not in training or fighting, but the other men keep theirs on all the time, or rolled down to their waist. The suits are "fully functional" so keeps some of the men entertained that way.
I do not recall any ground combat fights, though I am sure there were some, along with ships being damaged in space battles. Eventually after 1-2? years. the ship is rotated to a refit and recreation area.
The Hero looks for and finds the woman he came aboard the ship with. He is surprised she is obviously several years older than he is now. [18 each when picked up, but he is now about 20 and she is 25?] (Time dilation from his travels on the warship) She explains that while he was trained for fighting, the women were trained to provide R&R for all the men when the ships visited. The area is mainly bars and hotels and dining areas. She tells him that part of that training is the women are "programed" so that after a night with the men, the women become pestering/annoying/irritated with the man so he moves on to find another woman. (Prevents the men and women from becoming attached to each other) But she resists her programing and spends the whole ships visit with him. (a week?)
His ship goes off to fight again, and returns and she is now older again. (His 22 to Her 30) She is less able to resist her programing, but tells him the women have a plan to overthrow the aliens. Maybe he asks about kids and she tells him that all the R&R girls have been sterilized.
On the next visit to the R&R he is about 25 and she is in her 40s? She is no longer one of the R&R girls, but one of the organizers running the R&R area. [maybe called GreatMothers] but he finds her and she fills him in on the women's plans to take over and defeat the aliens. I think his refusal to wear the suit full time means he is less "programmed" than the other men and the women take him into their confidence.
I don't know if its on this visit or the next one the plan is implemented and the aliens are overthrown and they can return to their home planets.