I recommend you to stay next to public transportation hubs within the Beyoğlu district. This will allow you to take the tram, the metro or the ferry very easily.
In terms of location, the best is Karaköy: It is from there that you can take the ferry to Kadiköy and it is equal distance between the nightlife and the tourist attractions. I recommend you one of the following girl-friendly hotels there (click on the link to check the rates):
Upscale
The main issue with Karaköy is that it is a hipster district so if you don't feel comfortable with it, you may go around Asmali Mecet street in Galata. There is a funicular line from Tünel to Karaköy so it is still an excellent location. The view is also nicer.
Upscale
Mid-range
Pera Tulip Hotel
All these hotels are girl-friendly and near the nightlife. To avoid any issue, it is recommended to mention 2 guests at the time of the booking.
All these hotels are girl-friendly and near the nightlife. To avoid any issue, it is recommended to mention 2 guests at the time of the booking.