This looks like the marriage of Donald's parents, maybe not in exactly the same area but shepherds went where the work was, is this the marriage you have? This is near Glen Lyon (above the river Lyon) above Loch Tay near Kenmore.
http://www.rootschat.com/links/0vir/Name: Alexander Robertson
Spouse's Name: Katharine Campbell
Event Date: 03 Dec 1776
Event Place: Fortingall,Perth,Scotland
This could be Donald's ch not all children (particularly if residence was distant from church) were christened soon after birth, some were several years old and "done" in batches

Name: Donald Robertson
Gender: Male
Christening Date: 21 Feb 1790
Christening Place: FORTINGALL,PERTH,SCOTLAND
Father's Name: Alexander Robertson
Mother's Name: Catharine Campbell
From experience I would advise looking at the films of the parish records, time consuming but sometimes very worthwhile spending a day doing just that.
Re Rebekah: there are Butter's registered in Lethendy BUT where money was short girls often weren't christened or the parish clerk was absent minded forgot to record them or indeed just because Scots were misogynists!

I have a family link where I thought they hadn't had any daughters until by sheer chance I found one daughter listed on her brother's family stone and her name was the same as her mother. However there was absolutely no trace in the parish records, so I presume 2 earlier daughters named for the paternal and maternal grandmothers likewise hadn't been christened.
In Rebekah's case there is perhaps a wee clue from her childrens names ....... there is a Susanna Butter chr Fortingall 1793?