Maybe for your system, but that’s not always true. Is your GCC configured to use -march=pentium4 or -march=prescott by default? The output of gcc -v should answer that.
If GCC is configured with e.g. --with-arch-32=i686 (which is the case for several popular linux distros) then the default for -m32 codegen is -march=i686 which doesn’t include the SSE2 instruction set.
So this definitely affects GCC builds too.