I can tell from this that you are (a) male and/or (b) very very young. To have such an utterly limited view of what romantic relationships are; indeed to think that marriage is an exclusively romantic relationship, all the way through it from the starry eyed 20-somethings, through the child-bearing and child-raising years, to becoming grandparents together, to the 90-something couple supporting each other through mobility issues, illnesses and ultimately death, strikes me as having an absolutely arrested adolescent notion of what the marital agreement is and what it is FOR.
What it does do however is illustrate plainly why so many men cheat, and why so many men effectively abandon their children after marital breakdown, and then become father of the year to the next woman's kids. This view of marriage/monogamy (that it exists primarily to formalise/lock down sexual availability) makes that make perfect sense. You're not attached to the individual, the relationship isn't about forming and participating in a family; it's simply a way to guarantee regular sex. Without that there is nothing else of value there to differentiate it from a common friendship.
Women are cyclical creatures (so are men actually it's just your cycles are daily, rather than monthly or across a reproductive lifetime). If regular and consistent biweekly fucking is what you require, a monogamous long term relationship with a woman simply isn't for you.
I mean even pre kids, what about the week she's on her period? Should the allotted fucking continue that week, sore breasts, blood and all, because otherwise "the line will be crossed"? Or does she need to double up on the other three weeks to cover the missing week? Or perhaps it can be considered maintenance if she pleasures the man?
I suppose it goes without saying that biweekly sex should continue throughout pregnancy. Possible exemption for hyperemesis gravidarum.
Post natally: the NHS says 6 weeks minimum before resuming sex. Obviously you don't consider tiredness or hormonal loss of libido to be legitimate reasons to refuse sex; how about birth injuries? C section surgery?
Let's leap ahead to the 90 year old woman; her husband's libido is undimmed, meanwhile she has gone through the menopause, her sex hormones have all but vanished, she requires vaginal lubricant just to maintain a healthy vagina never mind to have sex, she's got arthritis. None of these things are susceptible to change. Still two times a week if that's what the husband desires?
And finally your idea that the person who has the issue with the situation as it develops ISN'T the one who should initiate the "difficult conversation" (which is what, exactly, from the woman's side, beyond exactly what they are already saying every night as "the hand" snakes across their belly at 11pm - "I'm tired, I'm not in the mood" - and you needn't think that is easy to feel, or to say, or that male partners make it easy to say repeatedly).
If you want to step out on your marriage, 100% it is on you to tell your partner that before you do, ESPECIALLY in a situation like this where sex is still being had in the marriage, however infrequently. Your wounded ego is not an excuse to lie by omission or to expand your partner's sexual pool without their knowledge (if they're sleeping with you, however rarely, they're sleeping with all the people you're sleeping with too). It's abusive.
Women are not robots. Marriage/monogamy is not purely about sex (at least not for women and not for the children of the marriage).
I think fundamentally women and men have very different views on this, which is why I pray to god my daughters end up being lesbians or at very least don't fall for the lies men tell about 'love'. They mean a very very different thing to what we mean by it.