There's nothing wrong with texting on HTC G2 vertical screen, but not on horizontal screen.

Have you installed a third-party SMS software? If so, uninstall it immediately. Maybe it's the input method. Try another input method. Are other software horizontal screens normal? Android system comes with screen direction conversion, and SMS is also a module that comes with the system, so there will be no problem in itself. If it cannot be sent, it is basically caused by a third-party module.