. Can we have anal sex if we dont want to have children?

No. Not in any Condition because Haraam is Haraam whatever your thinking is. If you do bad/prohibited thing with right intentions that thing will remain same, prohibited. Infact it will be a larger sin to do prohibited/Haraam thing to think that we are doing it with right way and right decisions. Whatever the way truth is Haraam will Haraam. So avoid it and don't call the wrath of Allah upon yourself. May Allah guide us all.