Any beam will bend under load. The amount of deflection depends on a number of factors, including
- the spacing between beams
- the spacing between supports
- the type and grade of the wood (or other beam material)
- the stiffness of the deck material
- the load (don't forget the table, chairs, grill, hot tub or wading pool, etc.).
You have to decide whether the amount of deflection you are likely to get when you use the deck is in accord with your sense of security (and that of your guests). Handbooks for this sort of thing are available at your library.
Your city building department may have recommendations or requirements.
It really depends on what type of wood the 2x8 is made from and what your weight is.