But there is some Biblical foundation to this issue, is there not? The story of Onan?
Yes and no. Its not built on any verse/line because Catholics don't do that. We do not (like Protestants do) read a verse and then (on our own) come up with a teaching/doctrine around what we think the verse says. We don't make up any teachings based on what we read.
The teaching is already there and has been since day one. We have what we call apostolic succession, in which, protected and guided by the Holy Spirit, the teaching that was taught before Christ by the old testament Jews and by Christ himself to His 12 is passed down to each generation. The bible backs up what they teach and is the evidence that this is what they always taught. It's true because the bible gives some evidence to it. It's not true because we read a verse and that's what we came up with.
So this, "every sex act must be open to life" is based on what was always practiced, taught and believed by the early Church/Christians and what was taught before that in the old testament/ Jewish culture.
The no condoms rule is because every single last sexual act preformed by married couples
must be at all times open to the procreation and transmission of new life... even if the act itself won't result in it. It does not mean you have to have a baby each time you have sex. It means that you can not put any type of contraceptive devices or chemicals in the sex act's way.
It also does not mean that you can not avoid pregnancy if having a child or more children is not prudent. It does not mean you have to live permanently celibate if you must avoid pregnancy. It means this; every sex act must end with [bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse] not outside the wife (which condoms are considered outside). So what a couple can do is not have relations during the times when the wife is fertile.
Its what we see the early Church teach and we see in scripture, like when Paul says to part for times of prayer. We see that this is what was always taught and we keep the teachings and we pass them down to each generation.