YABU – But only because imho the way you are looking at it, from your needs rather than your value.
This may go down like a lead balloon – but we are basically in a country of full employment. This is generally less 5% unemployment, which is seen as typically the lowest rate achievable; but to be fair there are countries with lower rates than ours (Iceland has 2.8%). At the moment good employees are expensive and hard to recruit – I spend many months finding suitable people, because in general the good ones already have employers who will try to keep them.
So, from your employers view they have (I presume) a hard working, committed, domain knowledgeable, skilled, company knowledgeable employee that helps them generate their targets. If you were to leave, there is a cost of replacing you – directly, through lack of the employee for a period of time, risk of finding another suitable employee and (based on the above) the competitive pressures that they face to get good employees. Every employee is in a strong position at the moment.
Ignore the fact that your wanting to change to a flexible working pattern – which as someone else has said is completely the future. The only thing that matters is would they rather have you on your terms, or can they get a better deal elsewhere.
Now, this is where you need to asses your own value. Are you confident you could get a new job to fit your desired balance (or become self employed and do the same) if you chose to leave? If yes, just state your demands and all the risk is on their side, and will likely accept. However, if you can’t really say so there is a band of negotiation, basically the risk elements that it’s worth giving you some flexibility for; and beyond this you’re being unreasonable.
So as long as your worth more than the next best alternative then point this out and just offer it as a take it or leave it, otherwise you need different expectations and you may have to accept your devaluing yourself as an employee by life choices; but not as a person as a whole, as you have a lovely DC as a result.