I just started a new yoga class. The instructor has us do a lot of slow forward bending while sitting. I already sit for long hours in front of my computer. Doesn't it seem like I should be doing more extension movements to counter act this?

You may want to ask your yoga instructor about the thinking behind these postures. In some forms of yoga, slow, forward bends are used to elongate the muscles and soft tissues before going into extension postures.

From a yoga point of view there are two reasons for this. The first is that sitting upright may put the hips in a flexed position but the back extensor muscles are working hard to keep the spine straight. Forward bending actually counteracts this upright sitting posture of the spine.

Second, lengthening the extensor muscles first allows the body to move into extension
without compressing the vertebrae of the spine. Most yoga instructors who use this method start with slow, relaxed forward bends and then move into extension postures. The emphasis is always on lengthening or elongating the spine and avoiding spinal compression.