They're two grown adults, it's not my business how they handle their partnership. If they have a pre-nup (I think palimony is the term), have an open relationship, etc... - who am I to judge? Just because I'm a straight, married (for love and not money), monogamous type - doesn't give me the right to impose that on everyone else.
The only relevance I see is in assessing Armstrong's character and make up. Macur did a good job spelling out the "dots" in her book; she leaves it to the reader to connect the dots. Outsized sexual appetite - check, using others - check, superficial charm, sense of entitlement, grandiose sense of self - check, check, check.
Given the depth of Macur's book, I don't see how a few tabloid photos add much to the conversation.
If the point is to 'hate on Armstrong,' I'd rather people just hate on him. But leave what he does with his partner out of it. Judging what people do in their bedrooms is a steep and slippery slope.