Mass-effect – cheat on the ME1 sweetheart in ME2 and keep our romance going in ME3

Even after her angry reaction towards Shepard's decision to work with Cerberus, Ashley sends him a mail apologizing for her outburst. In part, it reads:

I'm sorry for what I said back on Horizon. When I lost you two years ago, it tore me up. I prayed for you every day. I read a lot of Tennyson, thinking about you, just like I did when my dad passed. And then you came back, and it was like my prayers were answered. But I'm not who I was then, and neither are you.

I wouldn't have expected you to work for Cerberus, but I know why they sent you to Horizon. I saw how many people were lost there, and if anyone can stop the Collectors, you can. I can't go where you're going, but I can wish you luck.

Even so, she gives Shepard a very hard time on Mars, and in several exchanges between them you can try to convince Ashley that you did what you did for the good of the galaxy.

Shepard's also returned to working for the Alliance, and has more or less severed ties with Cerberus. It's pretty clear by the end of the Mars mission that Shepard is not on the best of terms with them.

I think taken as a whole, it's not too farfetched to conclude that Ashley's concerns about Shepard's temporary alliance with Cerberus have been overshadowed by Shepard's continual struggle against the Reaper threat.

You can choose not to romance anyone. That keeps things super-simple.

If, however, you'd like your Shep to get some play, you can totally romance Liara in 1 and then Tali in 2 and 3. Liara might be a little sad about it, but she's not going to hate you or anything as a result. I'm not sure if Tali says anything about Liara in 2, since I've never gone down that road, but given how other romances go about it I'd say Tali will ask you about Liara, and you are given the option of backing out or saying "nope, it's all you".

Basically, you won't be penalized if you start with Liara in 1 and go with Tali in 2 and 3. Liara's not going to be a pain and keep info from you or whatever over it. At most, there might be a snarky comment or two, but that'd be about it.