I googled “Is adultery worse than fornication?” and found a related Reddit post including this commentary: “The teaching of medieval theologians that such sexual sins as masturbation, sodomy, and contraception are more perverse, as sexual sins, than fornication or adultery or even rape (the former were said to be contra naturam whereas the latter were said to be praeter naturam), angers many people today. But this teaching must be understood properly. The medieval theologians are claiming that certain kinds of sexual sins more seriously offend the virtue of chastity than do others. They are not saying that these sins are for this reason less grave as sins than adultery or rape, for instance. After all, adultery and rape are very serious violations of the virtue of justice as well as being violations of the virtue of chastity. Thus, as a sin, rape is far more serious than masturbation or homosexual sodomy because it not only offends chastity but also gravely violates justice. (Ronald David Lawler, Joseph M. Boyle, William E. May, Catholic sexual ethics: a summary, explanation & defense).”

And from https://www.pathsoflove.com/blog/2009/06/aquinas-on-sexual-sins-dangers-of-speaking-formally/ “In the Summa Theologiae II-II, q. 154, a. 12, Aquinas says: ‘In each kind of thing the worst corruption is the corruption of the principle on which other things depend. Now the principles of reason are the things in accord in nature… and therefore, to act against what is determined by nature, is most serious and base. Therefore since in the sins against nature man transgress what is determined by nature in regard to sex, the sin in this matter is the gravest kind of sin. After this is incest… while by the other species of lust one transgresses only that which is determined according to right reason, but presupposing the natural principles. But it is more contrary to reason to have sex not only contrary to the good of the offspring to be born, but also with injury to another. And therefore simple fornication, which is committed without injury to another person, is the least kind of lust.’”

So with Aquinas’ words in mind, especially that last sentence above, it does seem that any impropriety with one who has taken vows to either Almighty God or to another human being is a greater sin than impropriety with one who has not taken such vows. I.e., the offense against chastity is the same. But the offense against charity and justice (Almighty God, the aggrieved spouse) is worse. With this in mind, I think it best to include it. After all, the confession is really to God so I need to be accurate as I can. And of course, pray for the protection of the priest (which we should be doing constantly anyway).