I suppose I would view it differently, depending on the reasons why...
If the person not saying it just doesn't feel comfortable saying it to anyone (as in LR's case), then it's part of that person - part of what makes them THEM - and it would be part and parcel of being in a relationship with them.
If he's not saying it because he's being asked to not say it to any other partners, I'd be concerned that his primary is somehow trying to control his feelings by controlling his language, and I'd probably talk with him about it (and maybe his primary, if they felt comfortable having that discussion).
That may not be what's going on here, but I'd definitely get that vibe until we had that talk.
Mono. Divorced, two kids (DanceGirl, 13; and PokéGirl, 11), two cats, one house, many projects.
My partner. Poly. In relationships with me, Xena, and Noa.
Poly. In relationships with Chops and Noa, and dating others.
Married, Poly. In relationships with Chops and Xena (individually).
Blog thread: A Mono's Journey Into Poly-Land (or, "Aw hell, there's no road map?!")
Slightly more polished blog with a mono/poly focus: From Baltic to Boardwalk