For me it comes down to children. Let's say you two get married and have children. What do you teach them, his beliefs or your own? Clearly you cannot teach both. Would you then choose to teach them nothing, thus minimizing the chance of them having any religious teaching? As a parent I want my kids to learn the same truths I hold dear, and if I had married someone of differing beliefs it would make this difficult.
Being in a relationship with someone of the same faith does not imply that those of other faiths are less important. It only makes for continuity in the family.