It is fair to point out PROVIDED you realize that isn't a good defense for Trump. Biden isn't the standard, he is the opposite of the standard for me personally. If most Christians were Biden supporters, it would actually make sense to play "whatabout'ism" with Biden. Unless I'm overlooking any Biden supporters here?
It's also true that Lot and his daughters were "from a different era" too, but God had already condemned incest and rape. So pointing out people are from different eras does absolutely nothing to defend the guilty party... so why bring it up if it does nothing to the context?
As for the comment(s) we are speaking about, take your pick... there are a multitude of them.
‘Is it wrong to be more sexually attracted to your own daughter than your wife’?
‘If I weren’t happily married and, ya know, her father...’
‘She has the ‘best body’... (in reference to Ivanka)
‘If Ivanka weren’t my daughter, perhaps I’d be dating her’
Now this one there isn't video/audio evidence like the ones above, but per WH aides, they said he regularly talked about what it would be like to have sex with Ivanka... John Kelly apparently reminded him that she was his daughter. Did Trump for sure say it? When we consider his previous comments, it's absolutely reasonable.