Martha getting engaged to Dr Thomas Milligan is just plain weird. Doesn't she love the Doctor? She's a human being. Humans have an amazing ability to cope with things and then move on. When she left the TARDIS at the end of series 3, she knew she needed time to get over her romantic love for the Doctor, but she didn't want to lose him altogether - thus the giving him her mobile. The fact that she fell in love with Tom Milligan is just another demonstration of how adaptable the human race is. It's not improbable at all.