This happened when I was 13/14 years old.
A friend of mine had a sexy mum, she always had a knowing smile which got my young dick so hard, and had large firm breasts too. Once when I was at their house one afternoon, it was only me and my friend, Dave, who were home, I went upstairs to go to the toilet and nipped into his parents bedroom instead-I gingerly opened the top drawer and had a quick rummage through Heather's underwear, sniffing a pair of silky black panties then rushing into the toilet for a quick wank, shooting thick strings of cum before rejoining Dave downstairs as though nothing had happened.
Anyhow, one hot summer afternoon my friend Ray and I decided to go to Dave's house. His mum Heather opened the door to us in a tight red t-shirt and it was perfectly obvious that she wasn't wearing a bra under it. Her large tits sagged slightly more than usual, and her nipples were visible in great detail. Heather told us that Dave had gone out to the shop and would be back soon, and that we could wait for him at the top of the stairs, as Dave didn't like anyone being in his bedroom when he wasn't there. Ray and I climbed the stairs and sat at the top, as soon as we sat down Ray whispered 'did you see her boobs?' I nodded and motioned to him to be quiet, as Heather was coming up the stairs. She gave us that sexy knowing smile as she walked past us, large breasts wobbling as she walked up the stairs and into her bedroom. She came back out a moment later with a white bra in her hand and walked into the bathroom, which was halfway downstairs on a landing, directly opposite from where Ray and I sat.
Heather went into the bathroom and flicked the door closed behind her, but not hard enough, the door was still open about a third of the way, enough for Ray and me to see her grip the bottom of her red t-shirt and pull it up over her tits, which spilled out and juddered as she pulled the tshirt over her head. Heather was standing with her back to us, but at a kind of 45degree angle, so we could see her left breast clear as daylight.
Already my teenage cock was rockhard and straining against my underwear, Ray must have been the same. As we watched in sheer sexual lust, Heather put the bra on, fastening it at the front then turning it round and pulling it over her tits, her thick nipples and wide aureola clearly visible to us. She then pulled the tshirt back on, exited the bathroom and went downstairs with another cute smile at us.
Dave came home not long after, we hung out for a while, then went our separate ways. I rushed home, into my bedroom, pulled my trousers down and frantically rubbed my stiff penis, the images of Heather's large firm tits engraved on my mind. I came a huge amount of thick white spunk, and masturbated to the memory many times after that.
A few days later, I spoke to Ray, and he told me that he too had wanked furiously when he got home.
Even now, I still occasionally think of those lovely tits while I'm masturbating.