Venting Bathroom Exhaust Fan

You have two big issues here.

First is that you have a serious risk of venting sewer gasses into your home. This is why drain vents open above the roof line or use one-way air admittance valves. This is likely to be unpleasant and possibly unhealthy.

Second is that a drain vent is typically too small and too moist to properly vent a bathroom fan. Your fan is probably designed to use a 4" duct, and instead it's going through a 1.5" diameter pipe. And if any lint/dust gets pushed out by your fan, it risks accumulating on the sides of the vent pipe, potentially blocking the pipe and slowing both your drainage and ventilation.

Beyond those functional problems, you'll also have a pretty visible issue that anyone inspecting your attic will see. You should do this the right way: add a properly-sized duct line and a roof vent for ventilation, and re-seal your drain vent pipe.

You should not vent out the soffit if you have soffit vents. The moist air will be sucked into the soffit vents, and cause just as much (or more) trouble as venting directly into the attic.

The best option is to vent out the gable end of the attic. Make sure the ducting is sloped slightly back towards the fan, so condensate can drain. Use rigid metal ducting, not that flexible plastic junk.