It’s obviously inadvisable to sit next to the opposite gender even if it’s not as obvious as not eating pork. Have you ever heard Lashon Hara (without wanting to, of course)? Does that make it okay? Obviously not.
And, incidentally, (perhaps?) you should not sit next to the opposite gender on a bus or train (even IF it is not technically forbidden? Ask your LOR, but don’t assume and extrapolate when it is not based on halacha.
As far as concerts, while it might be just fine to sit next to your spouse at a concert, that doesn’t mean there are no other issues in mixed concerts that need to be addressed. Are there any issues that concern you, personally? Perhaps ask your LOR.