By my reckoning, for straight male shep you have these options:
- Gianna Parasini (I guess a kiss on the cheek counts)
- Diana Allers
- Samara (??)
It seems to me that in order to have the maximum number of encounters, you have to carefully juggle the more "romantic" options while throwing in the more "casual" options whenever possible. Here's how I think it could work out.
- Express dissatisfaction with Sha'ira's gift.
- Romance both Ashley and Liara on the ship, pushing to the point where they confront you, and choose Ashley. Liara keeps a crush on you.
If Shepard has been wooing Liara and the human love interest at the
same time (or if Shepard picked a bugged renegade option in not
showing interest in Liara), after the next completed mission on a
primary world, the Commander will suddenly be confronted by both
potential love interests after the debriefing. They will ask Shepard
to choose between them.
- Consummate with Ashley as soon as possible, but keep being as flirty as possible with Liara.
- It seems that Kelly can be had "on the side" of whatever else you do. (correct?)
However, both male and female Shepard may also have romantic
interactions with the following characters [Kelly, Samara, Morinth], which do not prevent the
romancing of the main romance options but will not be rewarded with
the Paramour Achievement.
- You can have a quick fling with Jack early on.
- Because of a bug (maybe), you can romance Miranda and Tali and Jack after beating the game, by sequentially breaking up with each and moving on to the next one.
- Help out Gianna and get yourself that kiss!
- Romance Samara as far as it will allow you to.
- Reaffirm your relationship with Liara in the Shadow Broker DLC, but then go right back to what you were doing with whoever on the ship.
- Take every opportunity with Diani Allers before locking in with anyone else.
Sometimes Shepard can carefully time dialogs to bring about multiple
liaisons, particularly with non-squadmate characters such as Kelly
Chambers and Diana Allers
- Have Kelly on the side by meeting her on the Citadel and locking in with her first. The Romance Guide seems to support this:
Kelly Chambers: After Priority: The Citadel II is over, speak to her
once after you complete each "main story" mission. Eventually,
slightly before you are required to begin Priority: Rannoch, Kelly
will question Shepard about a significant other; if Shepard replies
that Kelly is that person, they will share a quick romantic encounter
in the cabin. If you wish to two-time with Kelly and another romantic
interest, you must "lock in" with Kelly first.
- Kiss Samara in the Citadel DLC, but don't pursue any further (maybe even break up? Is this possible?)
- Eventually lock in with Liara to finally consummate with her.
It looks like the only person you can have multiple unique encounters with under this strategy is Kelly, since she is so casual and in two games. The others you'll have to settle for the once.