People figure this out, and they weigh their options, desires and feelings along the way. Some dual-military couples go for a full dual career ride (DH and I did 26 years apiece, offset a bit), some go a few years and one gets out and may/may not go Reserve, some have kids, some don’t.
Over the period of the time since the early 70’s, when women started entering the service in greater numbers, policies were created for maternity leave, maternity uniforms, serving while pregnant, co-location (Navy will co-locate if possible, but not guaranteed), career breaks, adoption, on-base child care, etc.
Just as there are all kinds of relationships, there are all kinds of ways to be married and have a family while serving.
Neither my DH nor I needed to be joined at the hip in our relationship, and we occasionally chose tours apart that were right for our respective careers. We worked hard to see each other on a regular basis, and focused on clear and loving communications. It helped we are both strong, independent, “doers.” After 37 years, we look back on our time in uniform and feel the dual career challenges were well worth it and working together to get through them strengthened us. Absolute trust is a must.
We know many dual couples who were awesome at the kid-management side of things.
Everyone has their own way of traveling the path. As I said at the top, people figure this out.
To the OP, you have no idea how your life will unfold or what obstacles you will have to fight through, go over or around. Worrying about something so far in the future should not deter you from taking a step on the path to service. None of it happens all at once. There are exit ramps along the way. If you decide to leave AD after your obligated service, you are not teetering at grave’s edge age-wise, and have plenty of time to start a family or choose to be with the one you have more.
In terms of entering a Service Academy and having a relationship with someone in another service, there are numerous threads here on SAF. Search on “fraternization.” Some relationships are not, by the regulations, approved.