My whole life I looked with some amazement to people who ARE somebody, who without any hesitation said "I'm a Christian", "I'm a marxist" etc.
The same with "I'm a Theravadin"
After I converted myself years ago to a buddhist and found myself most at home in Theravada, I sometimes say "I'm a Theravadin". But more and more: "I'm a Buddhist"
Theravada is is my homebase, but I like to read Mahayana-texts too, that is: some, that resonate deep in me, even if doctrinair they are not hundred procent according Theravada-views.
For example I like to read the Vimalakirti Nirdesa Sutra, except the Sariputta-bashing of it.
It will not surprise me if in the future, and especially in countries that were not traditionally buddhist in any tradition, there will be a buddhism that is a mixture of Theravada and Mahayana.
So to answer the question: the proper attitude for a Theravadin towards Mahayana is trying to understand Mahayana or at least some Mahayana-schools because "Mahayana" because the differences between these schools are great.