A line though one of the parties' father's names can suggest illegitimacy but a line through both indicates that the clergyman did not think such details were worth recording. I have encountered this same problem in my own research.

If the marriage occurred in a church or chapel, seek out the church or chapel register - many places of worship continued, especially in this early period, to maintain their own registers as well as the Registrar's registers. It is possible (though admittedly not very likely) that there may be more detail in this second register. Most such registers have been deposited with county archives. Examine the entries adjoining your ancestors' - do they all have lines through these fields? If only some entries do, then check the name(s) of the clergymen - a pattern may emerge of one clergyman not bothering with these details.

If banns were called then seek out the banns register also.

Consider researching all the families of this surname in these two parishes, abstracting all occurrences from the 1841 and 1851 Censuses, and all the parish records for some decades prior to your ancestors' marriage. This should enable you to dismiss some families from being contenders and with luck may lead you to your direct ancestors. Almost certainly you will also have researched families of your ancestors' siblings, expanding your tree in doing so. A quick check on freebmd indicated that the surname Matthews (plus variant spellings which you'd also have to include) is not uncommon so this would be a substantial project. I've successfully followed this strategy with very commonplace Welsh surnames in rural parishes - so there is hope!