No, it's not unprofessional to work standard work hours. But your company's management may not see it that way, and thus it's up to you if you want to yield to their perception of what 'normal' is or whether you want to go work somewhere else. That is a personal choice that no one can make for you. There is a number of factors at work in your situation that I suggest you consider as you make that choice.
Growing company
In my experience start-ups tend to work hard/play hard. They work long hours and then throw expensive Christmas parties, or take their employees out to expensive dinners. Large companies, on the other hand, tend to work more standard 7.5-8 hour days, and look for across the board cost cutting strategies. The reason is that the latter pace is more sustainable in the long term. While startups work to get some market share at all costs, large companies think more about business continuity. As your small company grows, it will likely adopt this behaviour as well, but this doesn't happen overnight - it will change as new people come on-board and management will face new/different challenges. You may be mis-matched with your company culture now, but perhaps this will change in 6 months?
Hours worked and professionalism
These things are orthogonal. I've known people who killed themselves with 16 hour days but were still a very long throw away from being good 'professionals', while others who come in at 9 to leave at 5 did great work and made great long-term contributions to the company. If you want to make sure that you are being 'professional' then you should focus on how you are working, rather than for how long. Make sure your contributions and your attitude is worthy. Ultimately this feeds back into having good key performance indicators (KPI) such as bugs closed, issues resolved, features implemented, etc. Hours worked mean nothing, what matters is amount of work done. If someone ever challenges your contributions, you need to be able to speak about your KPIs, and ask them which one it is that they would like to improve. Perhaps challenging their idea of 'working hard' is easy based on the above, or it might just go over their heads.
Company Culture
Despite all the research and articles demonstrating to the contrary, a lot of managers think that working teams for long hours is a good idea. If this is the widely accepted norm in your company, you may not be able to change it. But keep in mind, this doesn't mean that there is something amiss with you, your level of dedication, or your professionalism. Some companies are just 'broken' (11 hour days as 'standard' rather than exception is a symptom of broken processes). If this is the case for you, you face a choice: accept it, challenge it, or leave. It's a personal choice, no one can make it for you. This ultimately feeds into....
Market value
How much do you value your time, and how much does your company value your time? If you are 'professional' you should be in a better bargaining position. A lot of very intelligent and driven people end up having families and other interest in life. If a company ever wants to have these sort of people work for it, it needs to yield to their demands of normal work schedules. More importantly, in my experience, people like that are able to spot each other. If you look around other companies in your area, you might be able to find just the right group/people that you would prefer to work with, and these people will appreciate your contributions and will not discount your contributions for not staying long hours.
Whether you want to wait or fight for your company's culture change or not is a personal choice. But to answer your question: no, working standard 8 hour days is not unprofessional.
