To be honest, this is an area of study that has yet to have been significantly explored, at least within a pre-modern context. The issue is complicated by the fact that prosecutions for rape in early modern Scotland tended to involve victims who were, in some way, perceived as extremely sympathetic. Thus, if you look at the record preserved in the National Archives of Scotland for justiciary court cases, you might assume that Scottish rapists had a fetish for pregnant women and minors. I must note, though, that my period of expertise is pretty narrowly confined to the sixteenth and seventeenth centuries, so I can't really comment directly upon the history of sexual violence during the period in which Outlander is set. That said, from my work with Scottish archival materials and from reading about the history of social violence in Scotland (for instance, you might want to check out Keith Brown's Bloodfeud in Scotland, 1573-1625: Violence, Justice and Politics in an Early Modern Society) I do think that presenting rape as an inescapable and constant threat to women in pre-modern Scotland is disingenuous and inaccurate. This is particularly the case when one considers the place that Scottish women occupied within their society compared to their English contemporaries.

Women in Scotland enjoyed greater marital freedoms, for instance. After the Reformation, they could divorce unfaithful husbands while retaining rights to the rents earned on any property set aside for their maintenance by their spouse at the time of marriage. Scottish women also were not barred from inheritance to the same degree as women in England and they could demand justice in their own name without necessarily depending upon the support and action of a male relative. Thus, when the third laird of Glenlyon attempted to rape the countess of Errol in 1587, she made a complaint of him to the Privy Council and he was subsequently summoned to appear in court. When he refused these summons, he was summarily outlawed by the crown. (For more on this, see John Hill Burton, et al., eds., The Register of the Privy Council of Scotland (Edinburgh, 1881), first series, volume IV, pp. 213-214.)

I'm sorry I can't answer you directly with regards to the 18th century but hopefully you can use some of this earlier information to extrapolate. And, hopefully someone will do a more systematic examination of the sources to tell us more in the near future.