Molly and Arthur i'm assuming just liked kids. My guess is that they always planne don having a family, but never really counted on it getting so big.

The war just helped them realise that if they ever want a family, now's the time, because they might not get another chance.
I can imagine Molly always secretly rooting for a girl, and having "Ginervra" picked out since Bill was born. She never thinks she has to use it, until she wonders why kid number seven looks different.
QUOTE(Dess)
What motivated Molly and Arthur to join the Order?
Well, Molly had lost her brothers, and they had principles, so maybe they thought that if they wanted to raise a big family, they needed a safe world, and they had to get it fighting.
Dess, i agree. That and the fact that Arthur and Molly are just good-hearted people. They would do what's right, even though Molly was frightened at the thought of losing her family.
This reminds me-we know Arthur and molly were in the Order during the Second war, but in the First war, we know Molly wasn't in it, due to Remus saying in OotP, something along the lines of "You wern't in the order then, Molly, you don't know what it was like". So, question:
Why wasn't Molly in the Order during the First War? Did Arthur decide it wasn't safe for her? Did she pull herself out to look after the kids? I think the first is more likely. Molly wouldn't want to look after the kids like an Army-Wife, wondering if her husband will come homw. She'd want to be out there doing something, so of course Arthur would have to put down a few ground rules.