Warning: open relationship that some readers might see as cheating.
Before you begin reading, please, keep in mind that this books is, first and foremost, a military sci-fi, not a romance.
There is an established couple that struggles with the situation where one of the partners (the MC) is home only 2 weeks out of 10 (hence open relationship, both men are OK with it).
There is a father-son relationship where the father does not approve of his son's (MC's) partner and his choice of profession.
There is a war going on and the MC is separated from his Shield (spec.ops) unit, forced to work with and rely on a different brunch of military. More conflict/interaction with his father ensues as they are forced to work together to collect intel on the alien enemies.
Finally, at 70% another MC (I know, I know, late, right? but it's a sci-fi, not a romance, remember?) and another POV emerges, and if you are looking for a passionate turbulent relationship, this is the part that rules :D