Family stability should be judged on a case by case basis.
100% agree also. You can't simply ban things because of any unproven assumptions. They either need to be empirically examined or given a go.
To be honest, some homosexual couples may not be suitable for IVF (as some straight couples may not be), and the only way you're going to know is by judging on an ad hoc basis like you say.
My inner lover of freedom stops short at giving IVF to everyone, really, unless they can show that they're not hopeless, and that goes for straight couples, gay couples, and singles. I was essentially raised by a single parent, so I can say that I turned out 'better' than many straight-couple parents. I would love to have a kid one day (although I'd adopt) but I'd like to think that me and my partner would think it all through. Shit happens in relationships, but you have to deal with them.
If the IVF system becomes more over-regulated people will just turn to other means, I would think. Luckily the cost of on-going IVF is at the moment sort of a would-be parental filter, unlike sex, in the mean time.