shown or not, if they (landlord and landlady) have sex off-screen, is the same thing.
You should consider sexless marriage, there are many ways you can go about it,
the landlord can have a lover, not be interested in the landlady anymore etc... many ways that don't imply a LI having sex or be involved with someone else that is not the mc.
When a li has sex with someone else who is not the mc is considered ntr to most of us, whether it is shown or not... it is still the same thing because of the jealousy factor.
So you mean they will be just married but not together in any form?
You should explain it better then, because your comments so far hinted at ntr (them having sex...).