My take on things

Gilles is definitely the babys father ,(the date on the divorce papers show the baby was conceived later) and i believe the scene with the american to be too fantastic to be true and more a manifestation of marions desires (she enjoys giving herself ) - the scene also underlines the lie that is underneath the marriage.

I believe gilles never really loved marion , or at least not for long - note when making love to his girlfriend in sardinia she is clearly playing the role of someone else for him (someone who likes to be taken?) So ultimately they are both sexually compatable and marions persuit of gilles results in both parties thinking they had found their ideal partner. This would also explain how gilles final sexual encounter with his wife was one of him forcing himself on her . (this was clearly a major part of their sex life)

As for gilles behaviour towards the baby , i believe its more him being ambivalant and inconsiderate to MARION , and at this stage he clearly realises he doesnt love her (but he does grow to love his son) This looks like the reason they stay together. As is the case of many marriages.

The most interesting scene is the dinner party where gilles seems to revel in his cruelty towards his wife , Marion enjoys the company of the brother and his lover and wishes her marriage was based more on unconditional acceptance and love like they share. Gilles rubbishes the notion completely and then humiliates her with the orgy story . Here we see the difference in their attitudes towards love , something obscured in the early/later chapters and as in life only found through time.

So what to make of the hotel room scene ? Why is marion in the room with him ? Does she hope to reignite their relationship and perhaps the humiliation of the rape finally gave her the courage to end things ? For gilles it appears an attempt to establish his dominance after the divorce but then HE appears to look for a reconciliation ..

In conclusion although gilles became bored of his wife he is willing to settle for this rather than continue his search for his ideal partner (which he probably realises now will be futile) while his wife who invested most in their marriage (her own parents conflicts and resolutions prob had a lot to do with this) has finally rejected it and gilles and found the strength to carry on her search . Ozons point being none of our searches are ever over and it is impossible not to cause and recieve pain along the way ....