Assuming the account is historical, perhaps their motivation was that because they were homeless, they saw themselves as ineligible for marriage...at least of any significant status....both themselves and their father---their father had no one to carry on his name/line and instead resorted to such to create a new clan of which their children would be the leaders.
They knew he wasn't the last man left, they could see a city from the cave. And they had to have known of Abraham from their father. They just used an excuse to engage in perversion they learned about in S&G.
He didn't like it.
Or perhaps they didn't think their father would survive the journey to Abraham's location (since he was nomadic, they may not have known where he was) or perhaps they felt at the city they would be force to seek refuge at, they would be seen as slaves. If they had children, they could pretend to be widows.
Perhaps they did not want to return to Abraham as nonstatus females of a nonexistent family to be put aside if their father were to marry a new wife that bore him sons.
Bottomline is that so few details are given in the story, one could create a hundred or more reasons----some noble, some despicable, some reasonable, some irrational----if one cares to. Thus it is better, in my view, to refrain from judgment.