I don’t think sexuality can be turned off like a switch, after many years of a normal conjugal life with a partner, even if the situation is “irregular”. The other partner may not be Catholic and may have expectations. The lesser evil may be maintaining a healthy relationship especially of kids are involved, while the Catholic partner tries to convince the non-Catholic partner to regularize the situation. IMHO, the sexual relationship in this case, while “grave matter” may have mitigated culpability due to lack of full freedom of the will. It’s up to a confessor to judge each case. We need to remember that everyone’s capacities and response to temptation are different. From the strong, much effort is expected. To the weak, much grace given.
Each case has to thus be evaluated pastorally on an individual basis, preferably with some guidelines from the Magisterium in order to ensure at least some consistency across all dioceses, parishes and confessors.
The fact that a Catholic, or a Catholic couple enters a church and says to the priest “I/we want to change our situation”, is a great thing and we need to nurture and help them progress to a regularization of their situation.
The same situation may apply to the divorced and remarried especially if the divorced partner bore no guilt in the divorce (abandonment, infidelity, abuse), and remarried a long time ago. Immediate continence may simply not be a realistic expectation, especially if an annulment process takes a very long time. Of course a streamlined annulment process could be helpful.
These are all complex issues of course. The brokenness of our society knows no bounds. The Church’s important job of evangelizing and converting all of us over to Jesus requires a loving, not punitive, approach. Again, IMHO.