You're being sarcastic maybe? I would interpret it as really selfish if my partner put his personal fitness as a priority above our family functioning.

No, I'm not being sarcastic, nor did I suggest that he put his personal fitness above his families functioning.

That said, I wouldn't enter a partnership with someone who did not respect my need for an hour or so of exercise 3-5 days a week and of course I would make the same effort to provide them that time as well, if they wanted it. In fact, I would argue that a persons physical fitness(to a certain level, of course) is crucial to a partnership and a healthy family. Full disclosure: my wife and I don't have kids- but these are my values and I'm proud enough of them to hope to bestow them on my kids if we ever have them.

I have a 2yr ol, and me and my spouse have the exact same way of functioning. We haven't quite figured out how to get 3-5 days but 1-3 days of exercise each is what we aim for. Or thinking it's the same: you can't have a happy family if your vitals (sleep, exercise, healthy food) are low. We try to combine 1 exercise (running) with going to a playground all of us and then one runs a round while the other cars for our son.

Oh! 3-5 hours a week. You said "extreme sports" or some such, and I was imagining several hours a day!

Endurance - not extreme. Middle distance running covers sprint type runs up to 3 miles. 5k runs (3.12 miles) take about a half an hour to complete for someone who is averagely slow (no offense to anyone who runs 5ks in 30 minutes). That means a 10k run takes about an hour to run. Cycling takes a bit more time to burn the same amount of calories, an equivalent ride to 10k(6.24 miles) would be about a 20 mile ride and would take the same person about an hour and fifteen minutes to ride. I really think this is a healthy way for someone to buy alone time within a family if you can't tell and I think that it's good for all parties involved.

If you're not attending to your personal fitness then the situation you're in is dysfunctional, regardless of whether or not it's a family, a childless relationship, or single life. So it's not possible to sacrifice personal fitness for family function because that is a core part of it.

If you're taking care of others to the exclusion of yourself, you're going to end up in a downward spiral of resentment. Nothing good will come of that.