Of course I agree that it is wrong to cause unnecessary harm to animals, but how can you say that am I causing them harm?

If you use or consume any nonhuman animals or their products, then you contribute to harming animals. Here is just one brief example:

When you consume dairy products, you support the forced insemination of cows to keep them pregnant and lactating as much as possible. Within about 24 hours of being born, calves are separated from their mothers, traumatizing both the cows and their offspring. The calves will be slaughtered for veal at around 3–18 weeks of age, used to replace their mothers for dairy production, or simply killed shortly after birth.

This process is repeated over about 5 years, or 3–4 lactations, during which it is common for cows to develop osteoporosis, as most of their calcium goes into producing up to ten times the natural amount of milk. It is also common for them to suffer painful mastitis in the udders. Once their milk production declines and the cows are “spent,” they are trucked off to be slaughtered for consumption. Many of them are pregnant at the time of slaughter.

The examples could continue on and on—the suffocation of fishes, the slaughter of animals used for clothing, the beating of trained animals for circuses, and so on—but the point is clear: harms like these are essentially unavoidable when we use animals as mere resources or things.