the fair use doctrine of copyright implies strongly that, if you own the physical roms, using an emulator to play them is fine legally, IE, not theft.
Abandonware approaches to Roms are dicey; some publishers have said that they don’t care, while others still protect the IP for later emulation issues.
Some, like Activision, actually sell disks with emulators on them… The Activision Classics for PS2 and XBox are an Atari 2600 emulator and a collection of Atari 2600 cartridge rom dumps from the original games. Atari also released an emulation pack for the NDS running their old arcade roms, and hacked versions of their roms, too…
So is the Namco Classics for GBA… except it is a MAME emulator!
Oh, and every infocom game ever released used an emulator; they wrote them, and compiled them to a virtual machine format, and the VM runs that code. The actual story file from any infocom game can be run on just about any machine that has a z-machine program for it… I’ve played my CP/M copy of HHGTTG on a dozen different computers over the years. Just by installing the correct Z-Machine virtual machine and the original story file.
And Java is a virtual machine system, too. Your Java plug in is an emulator for a machine which never existed: the Java Virtual Machine. By emulating a reference platform, the code becomes much more widely portable.
So emulation is not, itself, sinful in any way. What you run on it might be… either because you have no legal right to use the emulated code, or because the program itself is causing sin. But the same is true of every computer, game machine, and music player.