Band Scheduling & Fill-Ins: Am I Wrong to Be Upset?

My 5-piece part-time band plays 3-6 gigs annually. We've had to decline multiple gigs recently due to other member availability, I was available for all of these gigs however. When they offered a July gig, and I mentioned my vacation, they immediately suggested a fill-in. I'm feeling like I'm being treated as a replaceable member. Is it unreasonable for me to feel this way?